This package just contains placeholder objects to be used with the Highcharts charting library. Various charts in the Map Center can use these names for consistency with other modules. Each of these objects should be either null or an instance of Highcharts.Chart, which is further documented on the Highcharts site.
Object (instance of Highcharts.Chart) or null
The nhmc.charts.mainChart object is not currently used. It used to be in place to support an alternative chart view that could be shown instead of a map in particular cases (see a longer explanation with nhmc.mapCenterInit), but this isn’t currently done.
Object (instance of Highcharts.Chart) or null
The nhmc.charts.lowerChart object is used for supplemental charts that don’t need to be as large as the map itself. Right now this is used in one place: at the bottom of the sidebar in the Electoral College calculator to show how many electoral votes each candidate has in a given scenario.