barChart/bar/add - Adds a new bar to a bar chart.

Method parameters:
id - The ID of the bar chart to add the bar to.
value - The value of the bar.
label - The label of the bar.

Returns a JSON document of the following format:
<number> - The ID of the newly added bar.

Expected errors:
BAR_CHART_NOT_FOUND - A bar chart with the ID doesn't exist.