ScatterPlot - added variable size

ScatterPlots previously only supported a fixed/random size for points. This has been changed to support per-point variable size allowing for a third variable to be plotted along with the usual x, y variables.

The code permits either a series of sizes to be added, or for the size to be fixed for the whole series. From a statistical point of view, having fixed sizes for the points is preferable as, in my view, the current randomisation of point size suggests they represent a variable on the graph.