Once I had to plot the duty cycle wrt time. I just used a filter to get the voltage as the indication of the duty cycle and that did the job satisfactorily for me. For getting the frequency you can create a differentiator using Cadence's analogLib ideal components cap, vcvs, res, etc. and then add a filter to get a voltage proportional to the frequency in time. But this would only work if your time scale is very large and your frequency range is high to be satisfactorily seen. If not then you may have to revert to ocean and do some wave processing in code as suggested above. If you are not comfortable with Ocean, even doing it in MATLAB or something similar is another option.