adding control points to Tone Curve and other similar modules using the eyedropper
gimp does it, and it is very useful there, and I miss it in darktable. What I do instead is use the eyedropper to put a guide line in the Tone Curve, and the try and click on it. But this isn't as easy.
The same goes for the location of the control points in modules like Color Zones and Base Curve.