Add TODO

src/plot.c

@@ -1,5 +1,9 @@
 #include <plot.h>
 
+/*TODO
+ * Plot Circle
+ */
+
 double plot_calc_coordinate_x (double a1, double a2, float alpha, float beta) {
   /* coordinate transformation to transform 3D coordinates into 2D coordinates */
   double x = (sin(beta)*a1-sin(alpha)*a2)/(cos(alpha)*sin(beta)-sin(alpha)*cos(beta));