(e.g. <code>c(0, 100)</code>); <code>col_quantile</code> needs representative
numeric data; and <code>col_factor</code> needs categorical data.</p>
<p>If <code>NULL</code>, then whenever the resulting colour function is called, the
<code>x</code> value will represent the domain. This implies that if the function
is invoked multiple times, the encoding between values and colours may not
be consistent; if consistency is needed, you must provide a non-<code>NULL</code>
domain.</p></dd>
<dt>...</dt>
<dd><p>Addi