diff --git a/README.md b/README.md
index 13e4b8c..57f7b70 100644
--- a/README.md
+++ b/README.md
@@ -396,15 +396,19 @@ Demos are run by issuing (for example):
 These will run on screens of 128x128 pixels or above. The initial ones are
 minimal and aim to demonstrate a single technique.  
  * `simple.py` Minimal demo discussed below. `Button` presses print to REPL. 
- * `checkbox` A `Checkbox` controlling an `LED`.
+ * `checkbox.py` A `Checkbox` controlling an `LED`.
  * `slider.py` A `Slider` whose color varies with its value.
  * `slider_label.py` A `Slider` updating a `Label`. Good for trying precision
  mode.
- * `linked_slider.py` One `Slider` updating two others, and a coding "wrinkle"
+ * `linked_sliders.py` One `Slider` updating two others, and a coding "wrinkle"
  required for doing this.
+ * `dropdown.py` A dropdown list updates a `Label`.
  * `dialog.py` `DialogBox` demo. Illustrates the screen change mechanism.
  * `screen_change.py` A `Pushbutton` causing a screen change using a re-usable
  "forward" button.
+ * `primitives.py` Use of graphics primitives.
+ * `aclock.py` An analog clock using the `Dial` vector display. Also shows
+ screen layout using widget metrics. Has a simple `uasyncio` task.
  * `tbox.py` Text boxes and user-controlled scrolling.
 
 ### 1.11.2 Test scripts
@@ -766,17 +770,21 @@ Constructor args:
 
 The constructor displays the string at the required location.
 
-Methods:  
- 1. `value` Redraws the label. This takes the following args:
-    * `text=None` The text to display. If `None` displays last value.
-    * ` invert=False` If true, show inverse text.
-    * `fgcolor=None` Foreground color: if `None` the `Writer` default is used.
-    * `bgcolor=None` Background color, as per foreground.
-    * `bdcolor=None` Border color. As per above except that if `False` is
-    passed, no border is displayed. This clears a previously drawn border.  
- Returns the current text string.  
+Method:  
+`value` Redraws the label. This takes the following args:
+ * `text=None` The text to display. If `None` displays last value.
+ * `invert=False` If true, show inverse text.
+ * `fgcolor=None` Foreground color: if `None` the `Writer` default is used.
+ * `bgcolor=None` Background color, as per foreground.
+ * `bdcolor=None` Border color. As per above except that if `False` is
+ passed, no border is displayed. This clears a previously drawn border.  
+Returns the current text string.  
 
-If populating a label would cause it to extend beyond the screen boundary a
+If the `value` method is called with a text string too long for the `Label` a
+`ValueError` will be thrown. The width specified to the constructor should be
+sufficient for all possible values.
+
+If constructing a label would cause it to extend beyond the screen boundary a
 warning is printed at the console. The label may appear at an unexpected place.
 The following is a complete "Hello world" script.
 ```python
@@ -1368,6 +1376,11 @@ Methods:
     * `bdcolor=None` Border color. As per above except that if `False` is
     passed, no border is displayed. This clears a previously drawn border.  
 
+### Legends
+
+Depending on the font in use for legends additional space may be required above
+and below the `Meter` to display the top and bottom legends.
+
 ###### [Contents](./README.md#0-contents)
 
 # 17. Slider and HorizSlider widgets
@@ -1441,6 +1454,11 @@ user supplied args. The callback can be a bound method, typically of a `Screen`
 subclass. The callback runs when the widget is instantiated and whenever the
 value changes. This enables dynamic color change. See `gui/demos/active.py`.
 
+### Legends
+
+Depending on the font in use for legends additional space may be required
+around sliders to display all legends.
+
 ###### [Contents](./README.md#0-contents)
 
 # 18. Scale widget
@@ -2173,8 +2191,30 @@ The apparently obvious solution of designing a vertical `Scale` is tricky owing
 to the fact that the length of the internal text can be substantial and
 variable.
 
+## Screen layout
+
+Widgets are positioned using absolute `row` and `col` coordinates. These may
+optionally be calculated using the metrics of other widgets. This facilitates
+relative positioning which can make layouts easier to modify. Such layouts can
+also automatically adapt to changes of fonts. To simplify this, all widgets
+have the following bound variables, which should be considered read-only:
+
+ * `height` As specified. Does not include border.
+ * `width` Ditto.
+ * `rows` Height including borders.
+ * `cols` Width with borders.
+
+This support is fairly "micro" and does not take account of labels and legends.
+This means that `rows` and `cols` for `Dial`, `Meter`, `Slider` and
+`HorizSlider` do not necessarily reflect the full amount of space used by the
+control.
+
+The `aclock.py` demo provides a simple example of this approach.
+
 ## Use of graphics primitives
 
+See [demo primitives.py](./gui/demos/primitives.py).
+
 These notes are for those wishing to draw directly to the `Screen` instance.
 This is done by providing the user `Screen` class with an `after_open()` method
 which is written to issue the display driver calls.
diff --git a/gui/core/ugui.py b/gui/core/ugui.py
index b7c28fd..d29378b 100644
--- a/gui/core/ugui.py
+++ b/gui/core/ugui.py
@@ -550,6 +550,8 @@ class Widget:
         self.col = col
         self.height = height
         self.width = width
+        self.rows = height + 4  # For metrics. Default: allow for border.
+        self.cols = width + 4
         self.visible = True # Used by ButtonList class for invisible buttons
         self.draw = True  # Signals that obect must be redrawn
         self._value = value
diff --git a/gui/demos/aclock.py b/gui/demos/aclock.py
new file mode 100644
index 0000000..8a1cf1f
--- /dev/null
+++ b/gui/demos/aclock.py
@@ -0,0 +1,89 @@
+# aclock.py micro-gui analog clock demo.
+
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
+# Initialise hardware and framebuf before importing modules.
+import hardware_setup  # Create a display instance
+from gui.core.ugui import Screen, ssd
+from gui.widgets.label import Label
+from gui.widgets.dial import Dial, Pointer
+from gui.widgets.buttons import CloseButton
+
+# Now import other modules
+from cmath import rect, pi
+import uasyncio as asyncio
+import time
+from gui.core.writer import CWriter
+
+# Font for CWriter
+import gui.fonts.arial10 as font
+from gui.core.colors import *
+
+
+# Adjust passed Dial and Label instances to show current time and date.
+async def aclock(dial, lbldate, lbltim):
+    # Return a unit vector of phase phi. Multiplying by this will
+    # rotate a vector anticlockwise which is mathematically correct.
+    # Alas clocks modelled on sundials were invented in the northern
+    # hemisphere. Otherwise they would have rotated widdershins
+    # in accordance with maths. Hence negative sign when called.
+    def uv(phi):
+        return rect(1, phi)
+
+    def suffix(n):
+        if n in (1, 21, 31):
+            return 'st'
+        if n in (2, 22):
+            return 'nd'
+        if n in (3, 23):
+            return 'rd'
+        return th
+
+    days = ('Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun')
+    months = ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul',
+              'Aug', 'Sep', 'Oct', 'Nov', 'Dec')
+
+    hrs = Pointer(dial)
+    mins = Pointer(dial)
+    secs = Pointer(dial)
+
+    hstart =  0 + 0.7j  # Pointer lengths. Will rotate relative to top.
+    mstart = 0 + 1j
+    sstart = 0 + 1j 
+
+    while True:
+        t = time.localtime()
+        hrs.value(hstart * uv(-t[3] * pi/6 - t[4] * pi / 360), CYAN)
+        mins.value(mstart * uv(-t[4] * pi/30), CYAN)
+        secs.value(sstart * uv(-t[5] * pi/30), RED)
+        lbltim.value('{:02d}.{:02d}.{:02d}'.format(t[3], t[4], t[5]))
+        lbldate.value('{} {}{} {} {}'.format(days[t[6]], t[2], suffix(t[2]), months[t[1] - 1], t[0]))
+        await asyncio.sleep(1)
+
+class BaseScreen(Screen):
+    def __init__(self):
+        super().__init__()
+        labels = {'bdcolor' : RED,
+                  'fgcolor' : WHITE,
+                  'bgcolor' : DARKGREEN,
+                  }
+
+        wri = CWriter(ssd, font, GREEN, BLACK, verbose=False)
+        gap = 4  # Vertical gap between widgets
+        dial = Dial(wri, 2, 2, height = 70, ticks = 12,
+                    fgcolor = GREEN, pip = GREEN)
+        # Set up clock display: instantiate labels
+        row = dial.rows + gap
+        lbldate = Label(wri, row, 2, 100, **labels)
+        row += lbldate.rows + gap
+        lbltim = Label(wri, row, 2, '00.00.00', **labels)
+        self.reg_task(aclock(dial, lbldate, lbltim))
+        CloseButton(wri)
+
+
+def test():
+    print('Analog clock demo.')
+    Screen.change(BaseScreen)
+
+test()
diff --git a/gui/demos/active.py b/gui/demos/active.py
index fd37208..f3bb313 100644
--- a/gui/demos/active.py
+++ b/gui/demos/active.py
@@ -1,5 +1,8 @@
 # active.py micro-gui demo of widgets that respond to user control
 
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
 # Create SSD instance. Must be done first because of RAM use.
 import hardware_setup
 from gui.core.ugui import Screen, ssd
diff --git a/gui/demos/checkbox.py b/gui/demos/checkbox.py
index 1236a20..98a4714 100644
--- a/gui/demos/checkbox.py
+++ b/gui/demos/checkbox.py
@@ -1,5 +1,8 @@
 # checkbox.py Minimal micro-gui demo showing a Checkbox updating an LED.
 
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
 # hardware_setup must be imported before other modules because of RAM use.
 import hardware_setup  # Create a display instance
 from gui.core.ugui import Screen, ssd
diff --git a/gui/demos/dialog.py b/gui/demos/dialog.py
index 682de56..d1551fb 100644
--- a/gui/demos/dialog.py
+++ b/gui/demos/dialog.py
@@ -1,5 +1,8 @@
 # dialog.py micro-gui demo of the DialogBox class
 
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
 # hardware_setup must be imported before other modules because of RAM use.
 import hardware_setup  # Create a display instance
 from gui.core.ugui import Screen, Window, ssd
diff --git a/gui/demos/dropdown.py b/gui/demos/dropdown.py
new file mode 100644
index 0000000..29b464b
--- /dev/null
+++ b/gui/demos/dropdown.py
@@ -0,0 +1,45 @@
+# dropdown.py micro-gui demo of Dropdown class
+
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
+# hardware_setup must be imported before other modules because of RAM use.
+import hardware_setup  # Create a display instance
+from gui.core.ugui import Screen, Window, ssd
+
+from gui.widgets.label import Label
+from gui.widgets.buttons import CloseButton
+from gui.widgets.dropdown import Dropdown
+from gui.core.writer import CWriter
+
+# Font for CWriter
+import gui.fonts.font10 as font
+from gui.core.colors import *
+
+
+class BaseScreen(Screen):
+
+    def __init__(self):
+
+        super().__init__()
+        wri = CWriter(ssd, font, GREEN, BLACK, verbose=False)
+
+        col = 2
+        row = 2
+        Dropdown(wri, row, col,
+                elements = ('hydrogen', 'helium', 'neon', 'xenon', 'radon'),
+                bdcolor = GREEN, bgcolor = DARKGREEN,
+                callback=self.ddcb)
+        row += 30
+        self.lbl = Label(wri, row, col, 90, bdcolor=RED)
+        CloseButton(wri)  # Quit the application
+
+    def ddcb(self, dd):
+        if hasattr(self, 'lbl'):
+            self.lbl.value(dd.textvalue())
+
+def test():
+    print('Dropdown demo.')
+    Screen.change(BaseScreen)
+
+test()
diff --git a/gui/demos/linked_sliders.py b/gui/demos/linked_sliders.py
index 18e886a..2b8681d 100644
--- a/gui/demos/linked_sliders.py
+++ b/gui/demos/linked_sliders.py
@@ -1,5 +1,8 @@
 # linked_sliders.py Minimal micro-gui demo one Slider controlling two others.
 
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
 # hardware_setup must be imported before other modules because of RAM use.
 import hardware_setup  # Create a display instance
 from gui.core.ugui import Screen, ssd
diff --git a/gui/demos/primitives.py b/gui/demos/primitives.py
new file mode 100644
index 0000000..deddb54
--- /dev/null
+++ b/gui/demos/primitives.py
@@ -0,0 +1,45 @@
+# primitives.py micro-gui demo of use of graphics primitives
+
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
+# hardware_setup must be imported before other modules because of RAM use.
+import hardware_setup  # Create a display instance
+from gui.core.ugui import Screen, Window, ssd, display
+
+from gui.widgets.label import Label
+from gui.widgets.buttons import CloseButton
+from gui.core.writer import CWriter
+
+# Font for CWriter
+import gui.fonts.font10 as font
+from gui.core.colors import *
+
+
+class BaseScreen(Screen):
+
+    def __init__(self):
+
+        super().__init__()
+        wri = CWriter(ssd, font, GREEN, BLACK, verbose=False)
+
+        col = 2
+        row = 2
+        Label(wri, row, col, 'Primitives')
+        CloseButton(wri)  # Quit the application
+
+
+    def after_open(self):
+        display.usegrey(False)
+        # Coordinates are x, y as per framebuf
+        # circle method is in Display class only
+        display.circle(70, 70, 30, RED, 2)
+        # These methods exist in framebuf, so also in SSD and Display
+        ssd.hline(0, 127, 128, BLUE)
+        ssd.vline(127, 0, 128, BLUE)
+
+def test():
+    print('Primitives demo.')
+    Screen.change(BaseScreen)
+
+test()
diff --git a/gui/demos/screen_change.py b/gui/demos/screen_change.py
index ea15bfd..e766506 100644
--- a/gui/demos/screen_change.py
+++ b/gui/demos/screen_change.py
@@ -1,5 +1,8 @@
 # screen_change.py Minimal micro-gui demo showing a Button causing a screen change.
 
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
 # hardware_setup must be imported before other modules because of RAM use.
 import hardware_setup  # Create a display instance
 from gui.core.ugui import Screen, ssd
diff --git a/gui/demos/screens.py b/gui/demos/screens.py
index a9054d5..e4d6831 100644
--- a/gui/demos/screens.py
+++ b/gui/demos/screens.py
@@ -1,5 +1,8 @@
 # screens.py micro-gui demo of multiple screens, dropdowns etc
 
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
 # hardware_setup must be imported before other modules because of RAM use.
 import hardware_setup  # Create a display instance
 from gui.core.ugui import Screen, Window, ssd
diff --git a/gui/demos/simple.py b/gui/demos/simple.py
index 03c2c8d..37003f8 100644
--- a/gui/demos/simple.py
+++ b/gui/demos/simple.py
@@ -1,5 +1,8 @@
 # simple.py Minimal micro-gui demo.
 
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
 # hardware_setup must be imported before other modules because of RAM use.
 import hardware_setup  # Create a display instance
 from gui.core.ugui import Screen, ssd
diff --git a/gui/demos/slider.py b/gui/demos/slider.py
index c5e8190..b0543a4 100644
--- a/gui/demos/slider.py
+++ b/gui/demos/slider.py
@@ -1,5 +1,8 @@
 # slider.py Minimal micro-gui demo showing a Slider with variable color.
 
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
 # hardware_setup must be imported before other modules because of RAM use.
 import hardware_setup
 from gui.core.ugui import Screen, ssd
diff --git a/gui/demos/slider_label.py b/gui/demos/slider_label.py
index b020af9..425ea56 100644
--- a/gui/demos/slider_label.py
+++ b/gui/demos/slider_label.py
@@ -1,5 +1,8 @@
 # slider_label.py Minimal micro-gui demo showing a Slider controlling a Label.
 
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
 # hardware_setup must be imported before other modules because of RAM use.
 import hardware_setup  # Create a display instance
 from gui.core.ugui import Screen, ssd
diff --git a/gui/demos/various.py b/gui/demos/various.py
index 10c14ff..4271857 100644
--- a/gui/demos/various.py
+++ b/gui/demos/various.py
@@ -1,5 +1,8 @@
 # various.py micro-gui demo of multiple controls on a large display
 
+# Released under the MIT License (MIT). See LICENSE.
+# Copyright (c) 2021 Peter Hinch
+
 # Initialise hardware and framebuf before importing modules.
 # Create SSD instance. Must be done first because of RAM use.
 import hardware_setup
diff --git a/gui/demos/vtest.py b/gui/demos/vtest.py
index f3f36db..06cd5bb 100644
--- a/gui/demos/vtest.py
+++ b/gui/demos/vtest.py
@@ -56,7 +56,7 @@ async def ptr_test(dial):
                 ptr.value(v, RED)
             await asyncio.sleep_ms(200)
 
-# Analog clock demo. Note this could also be achieved using the Dial class.
+# Analog clock demo.
 async def aclock(dial, lbldate, lbltim):
     uv = lambda phi : rect(1, phi)  # Return a unit vector of phase phi
     days = ('Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday',
diff --git a/gui/widgets/checkbox.py b/gui/widgets/checkbox.py
index 580100f..f776471 100644
--- a/gui/widgets/checkbox.py
+++ b/gui/widgets/checkbox.py
@@ -11,7 +11,8 @@ class Checkbox(Widget):
     def __init__(self, writer, row, col, *, height=30, fillcolor=None,
                  fgcolor=None, bgcolor=None, bdcolor=False,
                  callback=dolittle, args=[], value=False, active=True):
-        super().__init__(writer, row, col, height, height, fgcolor, bgcolor, bdcolor, value, active)
+        super().__init__(writer, row, col, height, height, fgcolor,
+                         bgcolor, bdcolor, value, active)
         super()._set_callbacks(callback, args)
         self.fillcolor = fillcolor
 
diff --git a/gui/widgets/dial.py b/gui/widgets/dial.py
index eb21b47..11d061f 100644
--- a/gui/widgets/dial.py
+++ b/gui/widgets/dial.py
@@ -67,6 +67,8 @@ class Dial(Widget):
         self.pip = self.fgcolor if pip is None else pip
         if label is not None:
             self.label = Label(writer, row + height + 3, col, label)
+            #self.cols = max(self.cols, self.label.cols)
+            #self.rows += 3 + self.label.rows
         radius = int(height / 2)
         self.radius = radius
         self.ticks = ticks
diff --git a/gui/widgets/meter.py b/gui/widgets/meter.py
index 0ac9437..260cd12 100644
--- a/gui/widgets/meter.py
+++ b/gui/widgets/meter.py
@@ -23,13 +23,13 @@ class Meter(Widget):
             Label(writer, row + height + writer.height // 2, col, label)
         self.style = style
         self.ptcolor = ptcolor if ptcolor is not None else self.fgcolor
-        if legends is not None: # Legends
+        if legends is not None:  # Legends are static
             x = col + width + 4
             y = row + height
             dy = 0 if len(legends) <= 1 else height / (len(legends) -1)
             yl = y - writer.height / 2 # Start at bottom
             for legend in legends:
-                Label(writer, int(yl), x, legend)
+                l = Label(writer, round(yl), x, legend)
                 yl -= dy
         self.value(value)
 
diff --git a/gui/widgets/sliders.py b/gui/widgets/sliders.py
index 211ff93..8f7b8f6 100644
--- a/gui/widgets/sliders.py
+++ b/gui/widgets/sliders.py
@@ -64,6 +64,7 @@ class Slider(LinearIO):
             display.rect(self.slot_x0, self.slot_y0, slot_w, slot_len, self.fgcolor)
 
             txtcolor = GREY if self.greyed_out() else self.fontcolor
+            # Dynamic processing of legends supports greying-out
             if self.legends is not None:
                 if len(self.legends) <= 1:
                     dy = 0