You're plotting exactly what you're asking for. However, I don't think you're asking what you think you're asking. Your definition of PR1 doesn't appear to reference P0. It's a fixed scalar of 0.01;
did you intend to divide by 0.01? That's what 10*10.^-3 evaluates to. I would have expected PT there. Perhaps you intended 0.001? That's just '1e-3' in MATLAB notation (without the quotes).
Edit: Also, in MATLAB, log is the natural log (base e) usually written as ln. If you want the base 10 log the command is log10. dB is usually defined in base 10.