Create a dc Stacked Bar Chart

[Brian Lewis]

To support ; e.g. splitting by gender

[Brian Lewis]

@jeremy_kells @ghachey

As well as the stacked bar chart, some other minor changes in here: (Pull Request #259):

onChartClick binding to dc charts is now a callback binding &, supporting 3 possible arguments:

d3datum, data, key see dcCharts.ts for detailed description

I have made the relevant change in pie-base-chart and bar-base-chart

afterChartInit callback is reinstated!

filterHandler argument was unused and is gone - selected binding is gone.

To see the stack bar, uncomment <enrolment-district-chart in enrolment.cshtml; refer to enrolmentDistrictChart for setting up the layers.

[Brian Lewis]

Pull Request #259 closes