Clean up plotly code

[gramster]

Our current plotly code in charting.ts only supports line, scatter and heatmap charts, and does a fair amount of munging to go from the format of the data we pass to charting to the format needed by plotly. At some point we should clean this up and simplify/expand the interface to plotly. We also need to check the static chart generation; it may have been affected by plotly/plotly.js#446