a <code>qualitative_hcl</code> palette all have the
<em>same</em> luminance. Finally, <code>diverging_hcl</code> palettes use the same monotonic
luminance sequence in both &ldquo;arms&rdquo; of the palette, i.e., correspond to
two balanced sequential palettes diverging from the same neutral value.
The other two axes, hue and chroma, are used to enhance the luminance information and/or
to further discriminate the color.
</p>
<p>All three palette functions specify trajectories in HCL space and hence need eith