Extend Palette mechanism and offer more choices of palettes

[remifontan]

Extend Palette trait

I've been recently using plotters to do some scatters plots of point sequences. Its great, I love the gif backend. amazing crate.

As I wanted to colors my plots, I've been exploring the Palette trait.

I quickly wished for the following features:

• more choices of palette Following link has great examples of high quality palettes that could be used: https://bids.github.io/colormap/
• a way to index a palette color by float, in [0; 1] range
• a way to interpolate palette.

I'm not sure what would be the best way to implemented those features.. The Palette trait could be extended with functions like those

pub trait Palette {
    ...
    fn linear_sample(x: f64) -> RBColor;
    fn point_sample(x: f64) -> RBColor;
}

or perhaps a new struct could be implemented, InterpolatedPaletteColor, and its trait Color implementation would do the interpolation.

pub struct InterpolatedPaletteColor<P: Palette>(f64, PhantomData<P>);