I'm surprised you can see anything useful on that using plot rather than an image, but the plot function takes multiple arguments, as shown in the documentation:
doc plot
If you don't give it explicit x data then it will just use indexing so you can instead use e.g.
plot( linspace( 0, 1, 701 ), matrix_a(2:702,2:22) );
or whatever it is you want to plot on the x axis. I don't know what your actual frequencies are.
