After learning my way around the tone equalizer, I am still missing the interface of the tone curve.
Specifically, the following:
Tone curve allowed me to visualize the mapping as a univariate function, which I find very straightforward. The derivative was easy to read (very relevant for contrast).
Tone equalizer, on the other hand, is represented as deviations from the identity function, ie a 45^\circ line. I find it quite hard to visualize derivatives, so it is easy to get (nearly) non-monotonic mappings.
The tone curve allows arbitrary control points. This is needed very occasionally, but then it is quite useful.
I am wondering if it would make sense for the tone equalizer to provide an alternative interface along the lines of the tone curve as an option. Just to clarify, this is a proposal for a GUI element (mostly), not a fundamental change to anything the module does.