I'm not sure what you want. You can get a logspaced array like this: Assume that x is your xdata and you want the log array to have 20 values:
xlog=logspace(log10(min(x)), log10(max(x)), 20)
However, plot(xlog,ylog) will still look like plot(x,y), just the position of the markers differs. If you want to have the same optical output, you need to plot(log(x),y), but of course that will change the values on the axes (to the log values, obviously).