d luminance <code>l</code> (or equivalently
<code>l1</code>) are constants.
</p>
<p><code>sequential_hcl</code> codes the underlying numeric values by a monotonic sequence
of increasing (or decreasing) luminance. Thus, the <code>l</code> argument should provide
a vector of length 2 with starting and ending luminance (equivalently, <code>l1</code> and
<code>l2</code> can be used). Without chroma (i.e., <code>c = 0</code>), this simply corresponds
to a grayscale palette like <code>gray.colors</code>. For addi