Update 5 July 2012: Mason Green send me some info on an alternative to MathCad/MatLab: Scientific Python. He included a link to an excellent tutorial on Scientific Python. For an example of a practical application of Scientific Python, see Mason's PORC (Python Open Room Correction) project. Thank you Mason!
There have been several articles in Linear Audio supported by MathCad calculations and graphs; Burkhard Vogel's articles come to mind. If you are the curious type you probably discovered that a MathCad license can set you back $ 1500 or so. Not really DIY territory. However, here's a well kept secret: the student/educational version of MathCad is available for less than $ 100 and is identical to the expensive version, the only difference being that you don't get on-line support. And you need a professor- or student ID to be able to buy that version.
But it gets still better: if you buy Brent Maxfields'book Essential MathCad for Engineering, Science and Math for less than $ 50 you get an included MathCad 15 one-year student license for free. And the book is a great reference to the application so you can do without the on-line support. For beginners, I recommend Stephen P. Tubbs' MathCad for Electrical Engineers and Technologists. This last book takes you in slowly with calculating and graphing things like the current in a C-R circuit, for example. Both books are highly recommended! Note: the Maxfield book was published in 2009 so I don't know how many copies are still on offer.