Pressing <g> more than once toggles between the usual stereo, mono and single channel mode for calculating and showing the spectrum. <Pgup> and <Pgdown> adjust the frequency range. <Home> will set the frequency to 2756Hz. To half the resolution (and yet speed up the calculation) press <ALT>+<g>.
<Tab> change the palette of the graphical spectrum. <SHIFT>+<Tab> do the same for the standard spectrum analyzer at the bottom.
If you have difficulties interpreting this function here is a short explanation. The standard spectrum analyzer at the bottom shows you the frequency spectrum at the current moment. The higher a single bar, the louder the frequency. Now imagine looking at this spectrum from top, now every bar becomes a single dot. The height of the bar is now coded into different colors (from black low to yellow high). Now we can draw these point along the screen and see the spectrum as is progresses over time. This is somewhat a ``3D'' view of the spectrum, with the frequency coded along the y-axis, intensity coded in different colors and the time along the x-axis.