plot: consider implementing a gradient

[sbinet]

the original query had landed there: go-hep/hep#701

it would be great to allow some filling an area of a plot with a gradient.
e.g.: something along the lines of

as noted in the the original issue, there are already a couple of gradient implementations:

• https://github.com/lucasb-eyer/go-colorful/blob/master/doc/gradientgen/gradientgen.go
• https://godoc.org/golang.org/x/exp/shiny/iconvg/internal/gradient

[kortschak]

SGTM

[mazznoer]

Maybe my color scales library will be useful here.

https://github.com/mazznoer/colorgrad