Slowdown in Canvas draw


I experience a slowdown in the command canvas.draw().
I am drawing a moving bargraph by shifting the x-axis and adding a new bar.
The new bargraph is added every second, but starts to slows down after 5 minutes.

I monitored the time it takes to execute the canvas.draw() command and this increases from 200ms to over 1 second within the 5 minutes and therfore the delay since I am trying to display the new bar every second.

Any help appreciated!

# Using GTK
tyd = time.time()
print "canvas time =",time.time()-tyd