The LegendData structure is used by charts to describe the items
that should be displayed in an auto-generated legend.
A chart's legendData property contains an Array
of LegendData objects, one for each item in the Legend.
the size and placement of the legend marker.
If set, the LegendItem ensures that the marker's
width and height match this value.
If unset (NaN), the legend item chooses an appropriate
default width and height.