(required) a data frame, possibly an object of class rspec
that has wavelength range in the first column, named 'wl', and spectral measurements in the
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".