ACID test for different marker sizes + legend

Changing marker sizes in scatter plot and adding a legend ( issue #307 )
scatterPlotMarkers test updated
scatterplot legend: use marker color in legend

This fixes #307 and brings to light some other bugs:

 - some triangle markers are not drawn properly
 - in the relevant test, the XTick Labels are not used
Merge branch 'fix307': fixes #307