HCL-based Heat Colors from the colorspace Package
If package colorspace is available, its
heat_hcl function is used to generate a color
palette. Otherwise, the similar Heat 2 palette from R's own
hcl.colors are used.
This function was exported as
hcl.colors in surveillance
1.14.0 - 1.17.0 but is now internal to avoid a name clash with R 3.6.0
(or later), which introduced a function of that name in the base
the number of colors (>= 1) to be in the palette.
logical. Should the palette use colors? Otherwise grey levels are returned.