Allow anchoring gradients to Y values

[Panaetius]

For color gradients, it would be nice if we could say what Y-axis value corresponds to 0% or 100%, so there can be fixed colors for specific values even if the chart range is different.

For example, if I have two charts side by side, one goes from 50 to 150 and the other goes from 0 to 100, but I want e.g. 88 to have the same color value in both of them. Right now you'd have to dynamically create gradients for each chart based on their range, and recalculate them every time the series signal changes.