Add HSY histogram for curves and levels filters

[L.Sumireneko.M]

Hello @urzeye
Thank you for making this improvement!!
Why did I overlook this thread ever?

It has that MR (Merge Request) is ready, and there are also screenshots.
So I would like to add a vote.
I’m looking forward to it being merged.

By the way, I remember seeing a post somewhere on this forum about wanting a feature like this.
(I found the following thread,See @Heiny’s quetion 2)

Cross-channel color adjustment (especially hue/hue)

Dear developers, you implemented a great tool that you call “Cross-channel color adjustment”. Can I ask two questions? Is it possible to keep the (invisible) bezier handles always horizontal so that one can make a bell curve while keeping the rest of the line horizontal? In the current versions, any bell causes further (unwanted) “neighbour bells” because the bezier handles rotate away from their horizontal angle. Is it possible to replace that grayscale bar with a hue bar on any axis th…

So your improvement makes a lot of sense.
Cheers!