scatterplot: add control of points' stroke color
scatterplot: treatment of missing values in x/y axis scales

  - can use 'polylinear' scales as described at http://bit.ly/17g7nKw

  - but myscale(null) == myscale(0) so, if the observed values go below 0, you need to move the nulls to some special value.
Fix a problem with scatterplot y-axis scale
scatterplot test data: include missing values in another of the data columns