Per-datapoint styles

[SiegeLord]

E.g. points with varying size/color etc.

The most logical way to do it would be how we did the boxes_with_size, but it'll get unwieldy without default/keyword args. Alternatively, Color and PointSize options could take iterators... unfortunately default type bounds don't work so we can't make that work ergonomically. I really don't want to go the route of criterion etc where plots are done off structs.

See http://gnuplot.sourceforge.net/demo/pointsize.html for the implementation details. We'll be using lc palette for colors.