How do I reuse plots in matplotlib?

Here is one possible solution. I'm not sure that it's very pretty, but at least it does not require code duplication.

import numpy as np, copy
import matplotlib.pyplot as plt, matplotlib.lines as ml

fig=plt.figure(1)
data=np.arange(0,10,0.01)
ax1=fig.add_subplot(2,2,1)
ax2=fig.add_subplot(2,2,2)
ax3=fig.add_subplot(2,2,3)
ax4=fig.add_subplot(2,2,4)

#create the lines
line1=ml.Line2D(data,data)
line2=ml.Line2D(data,data**2/10,ls='--',color='green')
line3=ml.Line2D(data,np.sin(data),color='red')
#add the copies of the lines to the first 3 panels
ax1.add_line(copy.copy(line1))
ax2.add_line(copy.copy(line2))
ax3.add_line(copy.copy(line3))

[ax4.add_line(_l) for _l in [line1,line2,line3]] # add 3 lines to the 4th panel

[_a.autoscale() for _a in [ax1,ax2,ax3,ax4]] # autoscale if needed
plt.draw()