SetPlotAreaBgImage — Set a graphic file to be used in the plot area background
$plot->SetPlotAreaBgImage($input_file, [$mode])
SetPlotAreaBgImage sets an image file to be used as
the plot area background. The image can be scaled or tiled to fit.
$input_filePath to the file to be used. The file can be any type allowed by GD, which usually includes JPEG, GIF, and PNG.
$modeOptional display mode for the background image: one of the strings 'centeredtile', 'tile', or 'scale'. The default is 'tile'.
If a background image has been set, background color (set with SetPlotBgColor) is ignored.
Scale mode scales the supplied background image file to fit the plot area. Tile and centeredtile modes repeat the supplied background image file as needed to fit the plot area. The difference is that centeredtile offsets the start position within the background image by half its size, which works better for some images.
SetPlotAreaBgImage sets a background for the plot area, while SetBgImage sets a background for the entire image area. If both are used, the plot area background overlays that portion of the overall background.