Extend chart.ME to plot multiple portfolio symbols

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chart.ME can take advantage of a recently added feature of perTradeStats (see pull request #82), as anticipated by @braverock in comments to issue #42.

That is, add a chart.ME feature to produce a full scatterplot with respect to all the symbols in Symbol and depending on the combn.method passed to perTradeStats.

I volunteer to work on this, following the above-mentioned comments and what seems to be the original intention of @rossb34 in that issue.

Please note: there is a pending pull request on chart.ME (#80), what it would allow may be further comfortable to have when plotting multiple symbols trades points.