Concatenation - combining spectra lengthwise

If you have several independent spectra for which the velocity/frequency ranges are cover a wider range than any one such spectrum, it may be useful to combine them on a single plot. This is a trivial application, since all one has to do is increase the length of the x-axis of the final plot sufficiently to allow yourself room to plot all the spectra using new-plot and successive applications of overlay. There are however, a couple of more specialised applications, which we get to next.