Adds capability for colored scatter plots (issue #195)

Created by: Scorpiokat

I have closed the previous pull request, and created this one according to your comments @kortschak and @ctessum. Thanks!

@ctessum I decided to send this PR anyway (and it's up to you, obviously, to accept it or not right now), so that at least the plot has this capability and it will fix the #195 issue. Meanwhile, I will start working on the alternative solution which you have proposed. It should definitely look much more elegant and be an improvement of the existing way of making scatter, scatter colour and bubble plots.