(required) a data frame, possibly of class rspec, which
contains a column containing a wavelength range, named 'wl', and spectra
data in remaining columns.
by
number of spectra to include in each graph (defaults to 1)
scale
defines how the y-axis should be scaled. "free": panels can vary in
the range of the y-axis; "equal": all panels have the y-axis with the same range.
legpos
legend position control. Either a vector containing x and y coordinates
or a single keyword from the list: "bottomright", "bottom", "bottomleft",
"left", "topleft", "top", "topright", "right" and "center".