Change way that colours are defined in line charts

[fryford]

Too awkward to get the colour to match the series name each time

[henryjameslau]

Hmm, you'd want a way of being able to choose a specific colour for a specific line, a bit like datawrapper does it with the GUI, e.g. if you wanted to set all lines as grey but one or two highlighted.

[jtrim-ons]

I've hacked together a colour chooser, as I thought it might be helpful to just select the colours visually. I'm not sure if this is a sensible approach!

If this is useful, I'd need some help with the colour options.