This function can be passed to the labeller argument of
ggplot2::facet_wrap() or ggplot2::facet_grid(). Labels are processed
using parse_label().
Details
The label can include TeX math expressions (more common) or plotmath.
TeX math expressions are detected if more than two $ are detected in the
string. Parsing is accomplished with the TeX function from the latex2exp
package; a warning if thelatex2exp is not available.
Text to be parsed as plotmath is detected when the text begins with !!.
