SetBgImage — Set a graphic file to be used in the graph background
$plot->SetBgImage($input_file, [$mode])
SetBgImage sets an image file to be used as the graph
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 'centeredtile'.
If a background image has been set, background color (set with SetBackgroundColor) is ignored.
SetBgImage sets a background for the entire image area, while SetPlotAreaBgImage sets a background for the plot area (generally, the area between the axes). If both are used, the plot area background overlays that portion of the overall background.