Color gradient fill for area plots

[hkfb]

Support for coloring the area under the line plots, using color gradient corresponding to the value of the plot, using a specified color map.

The color map will contain mappings between continuous values and colors.

Example continuous mapping:

{
    0.0 : [100, 100, 0]
    0.5 : [100, 100, 50]
    1.0 : [100, 100, 255]
}

Example plot: