Matplotlib is used for plotting given data. NumPy is used to perform calculations to produce the data (which, in turn, matplotlib can plot it). However, you said that you have the equation in a string form. Are you looking to be able to evaluate any arbitrary parametric equation (given possibly by user input, perhaps?). If so, then maybe you need to look at Numexpr which can evaluate a string expression for you and return numpy arrays of the result.
Let us know if you have any questions.