Add documentation to a number of files.

 - components/baseAxis.ts
 - components/legend.ts
 - components/plots/abstractBarPlot.ts
 - components/plots/areaPlot.ts
 - components/plots/horizontalBarPlot.ts
 - components/plots/verticalBarPlot.ts
 - core/broadcaster.ts
 - scales/colorScale.ts
 - scales/interpolatedColorScale.ts
capitalize Listenable