The figure shows the colour map with low gradient ripples to indicate how it represents low-contrast regions in different locations of the colorbar, and a plot of the lightness difference, dE, indicating better perceptual-uniformity the flatter the curve.
→ Compare to the diagnostics of the rainbow (a.k.a. jet) colour map.
Diagnostics performed after Kovesi, P. (2015), Good colour maps: How to design them, CoRR, abs/1509.03700 and based on MatLab scripts from Peter Kovesi.
Read more about it here.