How to get this kind of plot

select a suitable colormap (You might have to define your own. See colormapeditor.) Let ix=1 be white.
map the frequences to the colormap-indicies, e.g [1:64] (should have been [2:64]). Assign 1 to missing values or whatever white means.
use image - not imagesc
fix the axes ticklabels

or better(?)

select the colormap of your choice and add an extra row for the "missing-value-color"
map frequency to the "colormap" and missing-value to the "extra row".
the colorbar ticklabels will require some extra work

I use the approach described above. Pro: I get full control of the colors and the resulting code is fast. Con: requires some coding and testing.

pcolor might offer a simpler way. Try

    x = [1:8];
    y = [1:8];
    z = x' * y ;
    z(4:5,7) = nan;
    pcolor( x, y, z )

Note, this gives a plot with 7x7 squares.

per isakson 2013-11-17

I updated the answer

Edward 2013-11-18

Thanks for the help, Ive accepted your answer!