d performs a least squares fit to either absorption
or emission data in the current buffer using Voigt profiles.
Lines must be <a href="active.html">active</a> (i.e. subtracted from the data to produce
a set of residuals) before they can be refined using the
<b>lsqfit</b> command.<p>

Lines which show appreciable ringing need a "filtered fit", in which
the residuals are filtered back to the cutoff set by the original
interferogram between each iteration; this process is invoked by the
lsqfit filter version,