How can I plot frequency vs magnitude of wavelet transform?
Continuous wavelet transform (CWT) is a method for time-scale analysis. Yes, you read it correctly, scale, not frequency. However, it is possible to map the scales to frequencies, and even quite easily. Since you are a MATLAB user, you will probably want to use this function, which does the following:


  F = scal2frq(A,'wname',DELTA) returns the pseudo-frequencies corresponding to the scales given by A, the wavelet function 'wname' (see wavefun for more information) and the sampling period DELTA.


As you can read, the frequencies are not the real frequencies in the signal, but approximations. If you want the real frequencies, you should be using the short-time Fourier transform (STFT).

So, when should one use the CWT? As a general principle, wavelets work well when the signals are transient (i.e. have quick changes).

EDIT: haha, I don't know what I'm doing here. sorry :-D completely missed the point of your question. Just read the title....