(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"