SetPlotBgColor — Set plot area background color
$plot->SetPlotBgColor($color)
SetPlotBgColor sets the background color of the
plot area of the image (the area inside the margins, title, and usually the
axes).
Background color is disabled by default. If you want a background color for the plot area, you must enabled it with SetDrawPlotAreaBackground.
Background image takes priority over background color. If a background image was set with SetPlotAreaBgImage, then no background color will be drawn, even if it was enabled.
The default background color for the plot area is white (if enabled as noted above). If background color for the plot area is not enabled, the overall background color (or image) will be seen in the plot area.