Just to add my perception of the feature as I see it and how it could/should be implemented.
I note that this threads last comment was over 2 years…hopefully a refresh and DxO team insight/comment might be forthcoming?
In the Tone Curve controls ~ a picker to select a point on the image that would place a fix on the curve line. This function ideally needs to allow for picking multiple points on the image so that when adjusting the curve, specific slight changes can be made to the Tone Curve based on those chosen points in the image.
TIA 
PS for more control ~ right clicking on a point on the curve allows its deletion/reset, if required rather than resetting the whole curve
PPS it was @platypus that pointed out to me that I was posting a similar ‘feature request’ here
Tone Curve…more nuanced control(s) - DxO PhotoLab / Which feature do you need? - DxO Forums