Generating the background of imatest color error chart generated by using color checker?

  adobe-illustrator, c++, cielab, colors, matlab

I am having some trouble finding the background of the following plot made using imatest. Basically what I want to know is that how, or from where, can I find the background of this plot. The imatest website mentions that the colors of the chart are generated at a constant Luminance L* = 70 and by varing a* and b* from -80 to +80. I have been looking for Lab color generator but all software generate colored points. But I want to get a continuous image by varying the a and b values. Any idea?

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Source: Windows Questions C++