set_data and autoscale_view matplotlib

I have multiple lines to be drawn on the same axes, and each of them are dynamically updated (I use set_data), The issue being that i am not aware of the x and y limits of each of the lines. And axes.autoscale_view(True,True,True) / axes.set_autoscale_on(True) are not doing what they are supposed to. How do i auto scale my axes?

import matplotlib.pyplot as plt

fig = plt.figure()
axes = fig.add_subplot(111)

axes.set_autoscale_on(True)
axes.autoscale_view(True,True,True)

l1, = axes.plot([0,0.1,0.2],[1,1.1,1.2])
l2, = axes.plot([0,0.1,0.2],[-0.1,0,0.1])

#plt.show() #shows the auto scaled.

l2.set_data([0,0.1,0.2],[-1,-0.9,-0.8])

#axes.set_ylim([-2,2]) #this works, but i cannot afford to do this.  

plt.draw()
plt.show() #does not show auto scaled

I have referred to these already, this , this.
In all cases I have come across, the x,y limits are known. I have multiple lines on the axes and their ranges change, keeping track of the ymax for the entire data is not practical

A little bit of exploring got me to this,

xmin,xmax,ymin,ymax = matplotlib.figure.FigureImage.get_extent(FigureImage) 

But here again, i do not know how to access FigureImage from the Figure instance.

Using matplotlib 0.99.3