The rgbalpha plotting style assumes that each pixel of input data contains
an alpha value in the range [0:255]. A pixel with alpha = 0 is purely
transparent and does not alter the underlying contents of the plot. A pixel
with alpha = 255 is purely opaque. All terminal types can handle these two
extreme cases. A pixel with 0 4#4 alpha 4#4 255 is partially transparent.
Terminal types that do not support partial transparency will round this value
to 0 or 255.
