Remaining chart issues (possibly implement)

[grovduck]

These were low priority items left over from #3. Opening an issue just to track them ...

Chart enhancements:

• Add additional series with average (for continuous variables) or sum (for area variables) across all series currently shown along with checkbox option to not show it
• Capture area variable as (pixels | acres | hectares | sq. km) with pulldown for units choice
• Allow option to change between line charts and stacked bar charts
• Add percentage as a statistic (variable)