Support multiple (line/scatter) plots in one chart.

[OliverColeman]

It would be lovely if it was possible to produce a line/scatter plot for multiple series of data in the one chart. I noticed that AbstractChart actually has support for up to 20 series of data, but XYChart currently only supports using two of these for X and Y values respectively. Such a chart would be more complicated, for example: what if series have different scales/meanings and so necessitate multiple axis labels; and should it be possible to use more than one series for (say) the x-axis (e.g. allow arbitrary pairings of series for any number of XY pairings)? I suppose there are examples in other charting libraries about how to handle such things, but perhaps it would take substantial work to realise something nice.