Resizeable, much taller histogram for levels / colors adjustments (and maybe a linear vs logarithmic view toggle)
Complementary/applicable to #4994, but first and foremost something I'd like to see in the "Adjust" dialog: the histogram there is way too small for me to be able to perceive subtle details, whereas that dialog is using merely one half of my screen's height (1080px), and the histograph itself is using only 100 pixels fixed height, and this is a problem for me, because as you can see in this very common situation in under-exposed low-key photography, I can't accurately tell where the end of the highlights is by looking at Shotwell's histogram, because the limited height of the widget doesn't allow enough vertical resolution for the histogram to show subtle values:
In contrast, with GIMP's resizeable "Levels" dialog, I can have a very tall histogram that gives me absolute certainty that I'm clamping to the right spot of the highlights, if measuring linearly:
Side-note: of course if you tell it to graph logarithmically, then it's a different story (as there are some very minor highlights information there that would be thrown away by my linear-based adjustment; however, in most cases logarithmic is "mathematically correct, but perceptually wrong", so I tend to just go with the linear representation):
What would be amazing is if Shotwell would also show a warning icon (with tooltip) when you clamp in a way that would lose information (mathematically, vs perceptually)...