Below, I've plotted an arbitrary function between arbitrary limits, -5 and +37.
x = linspace(-5,37)';
y = erf(x/20);
Now you asked for the x-axis to be in percentage. That's a little ambiguous, so I've intrepted that to mean percentage of the lower x-limit to the upper x-limit. (Note: that may not be what your data lies between.) I've used xlim to get those values.
xl = xlim;
xperc = (x-xl(1))/diff(xl)*100