The Chart control by default automatically creates one or more legend entries
for all chart types. The entries that are used for pie and doughnut charts are
the names of the plotted data points, while entries for all other chart types
are the names of the plotted series. These items are referred to as the
default legend items. Note that these default legend items can optionally not
be displayed.
A custom legend item is not associated with data, and is always appended to existing default legend items.