Support for Transparency in plots

[vsudbrack]

Hello!

It would be great to be able to control transparency (alpha) for plot elements, such as points and lines. I can think of implementing transparency in two possible ways. Either as part of the color specification, e.g. "red-60" to indicate red with 60% opacity (alpha = 0.6), adopting the style "colorName-alpha", or as a separate parameter in the plot functions, e.g. [float$ alpha = 1.0].

For example, consider a plot showing the evolution of a phenotype over time, where each dot is an individual and the line is the mean in the population at a given time:

In this case, partial transparency would help to see any internal structure within the dense clusters of individuals.

Thanks in advance for considering it :)

[bhaller]

Works nicely now AFAICT, let me know if you see problems. Thanks, good suggestion!