Add a vertical fill gradient

[IKupriyanov-HORIS]

Useful in some cases: https://stackoverflow.com/questions/58525699/how-can-i-add-a-vertical-colour-gradient-to-a-ridgeplot-in-ggridges

https://stackoverflow.com/questions/27250542/how-to-make-gradient-color-filled-timeseries-plot-in-r

[alshan]