oh...ok
1) So in the drawing, add geometries for the blue lines
sheet.<guide1>!Geometryn.x1 = sheet.<C1>!controls.guide 1 sheet.<G1>!geometryn.y1 =sheet.<C1>!controls.guide1.y
sheet.<guide1>!geometryn.x2 = geometryn.x1 sheet.<guide1>!geometryn.y2 = Sheet.<axis>!geometryn.y1>
A series of vertical lines from control points to axis
since different color...maybe a series of shapes in a group
2) set up actions so that right click can pick what you want
actions.C1 = setf(getref(actions.c1.checked), if(actions.c1.checked,0,1))
control point 1 behavior = if (actions.c1.checked,0,1) same for y
in vertical line geometrye…...noshow = if(actions.c1.checked, 1, 0) // it has inverse behavior
so user can right click and select "show C1"...that will show the control point and the vertical line.
user deslects by right click and noting a "check mark" next to "show C1"
you could try to play with a delta between "old C1" and "new C!" but be careful since the cells like user.deltac1X and Y
would be updated for every 0.0001 change in C....lots of cycles.
or use actions to turn on all "Cs"....when done, use actions to turn off all "Cs" veritical lines