I would appreciate a more comprehensive help text at the top. The code is documented pretty well. There are some coding styles that could be better (i.e. the while loops). Since you are plotting the asymptotes, maybe the function could return the break frequencies and the corresponding slopes of the asymptotes. It can have the same behavior as BODE, in which if there are no output arugments, then it would create a plot, and if there are outputs, then it wouldn't create a plot.
16 Mar 2007
Fix error: A reviewer pointed out that a certain transfer function didn't work with the current version.