Gradient AND Categorical fill

[DominiqueMakowski]

Hey, thanks for your awesome package.

I was wondering if it was possible to mix gradient and categorical fills.

Based on your vignettes, it would be like mixing the picture from Catalan regional elections and the one with Temperatures in Lincoln.

For example, having, for each "line" (y axis), two density plots (corresponding to two categories, such as in your example indie vs. unionist), but those two density plots are both gradient filled, with a different gradient. For example, the unionist would be "from dark blue to white" and the unionist "from white to dark red".

Is that possible? Thanks a lot!

[trevorld]

Do you have an example? Somewhat more advanced gradient fills may be possible after #98