BUG: Correctly check for empty data in ScatterPlot.map_index.
BUG: Handle NaNs in ScatterPlot.map_index.
Merge pull request #210 from epatters/fix-scatterplot-no-data

Handle empty/missing data in scatter plot
Added a line for the fix merged in PR #210.
Update dev_requirements.txt
Merge pull request #212 from enthought/update_changes_from_epatters_fix

Added a line in CHANGES.txt for the scatterplot improvement from PR #210.
Merge with master.