A bar chart illustrates comparisons among individual items.
Categories are organized vertically, values horizontally.
Categories can contain multiple data points. In this case, each bar is colored
according to the series that it represents.
Data Model
Mandatory channels: one value channel.
Optional channels: label channel, other value channels.
Each value channel is used for series values and the label channel
is used for categories.
Examples
One series chart with legend.
Two series chart with legend.
Stacked bar charts.
Wide bars, with width controlled by the axis gap attribute.
Bars with value labels, as controlled by the plot valueFormat attribute.
