| 
            plscolbga
          ( | r , | 
| g , | |
| b , | |
| alpha ); | 
Set the background color (color 0 in cmap0) by 8-bit RGB value and PLFLT alpha transparency value (see the section called “Color Map0”).
r
          (PLINTUnsigned 8-bit integer (0-255) representing the degree of red in the color.
g
          (PLINTUnsigned 8-bit integer (0-255) representing the degree of green in the color.
b
          (PLINTUnsigned 8-bit integer (0-255) representing the degree of blue in the color.
alpha
          (PLFLTValue of the alpha transparency in the range (0.0-1.0).
This function is used in example 31.