Drawing a diagonal (or any arbitrarily inclined and offset) line in Matplotlib

I wanted to overplot a simple 45 deg line on my data. As far as I know, Matplotlib doesn't accept equations, only sets of points, so here's a clumsy solution to it, based on this:

    import pylab as pl
    import numpy as np
    x1,x2,n,m,b = min(data),max(data).,1000,1.,0. 
# 1000 -- a rough guess of the sufficient length of this line.
#you can actually calculate it using the Pythagorean theorem.
#1, here -- the slope, 0 -- the offset.

    x = np.r_[x1:x2:n*1j] #http://docs.scipy.org/doc/numpy/reference/generated/numpy.r_.html
    pl.plot(x,m*x + b); pl.grid(); pl.show()