Two issues
- You never define your x variable, f
- Your code calculates a single value for Vrms
By default, semilogx does not use markers. Therefore, when you plot a single point, there is no line, so it appears as if nothing got plotted. Add a marker style to your LineSpec in order to see it.
plot(5,5) % no marker style
hold on
plot(4,4,'go-') % includes a marker style
hold off
legend
