When the legend background is opaque, you can use the BACKGROUNDCOLOR=
option to set its color. In the following example,
BACKGROUNDCOLOR=GraphAltBlock:Color for both the inside and outside opaque
legends. Other style references that you can use include GraphHeaderBackground:Color,
GraphBlock:Color, or any other style element with a COLOR= attribute. You can also
specify a specific color, such as BACKGROUNDCOLOR=white.
Using a Legend Title and Title Border
By default, legends do not have titles. To add a title, you can use the TITLE= option.
You can also add a dividing line between the legend title and the legend body with the
TITLEBORDER=TRUE ...
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