Suppose that I have a function f(x). You can think of f(x) as strictly positive if it helps.
Now, there is a secondary function, say M(x), which, for a given x gives a number.
I'd like to create a plot of f(x), except with the column from y = 0 to y = f(x) coloured with a colour determined from M(x). Moreover, I'd like the colours in the horizontal direction to be blended together (as in a gradient).