Plotting package?

[Andrew Koenig]

This may be a foolish question, but what's the most straightforward way to 
plot a bunch of data in Python?

That is, I want to write a program that does some number crunching, and then 
I want to change some parameters and watch how the changes affect the 
results.  I could produce a file to hand to gnuplot, but that's a bit of a 
pain; so I'm wondering if there is a widely used package to which I can give 
my x-y pairs and have it produce a graph for me with axes, scaling, etc.