python 2.7 mac osx interactive plotting with matplotlib not working

Question

From here I found this code:

import random
from matplotlib import pyplot as plt
import numpy as np
plt.ion() # interactive mode
ydata = [0] * 50 
# make plot
ax1 = plt.axes() 
line, = plt.plot(ydata)
plt.ylim([0, 100]) # set the y-range
while True:
 randint = int(random.random() * 100)
 ymin = float(min(ydata)) - 10
 ymax = float(max(ydata)) + 10
 plt.ylim([ymin,ymax])
 ydata.append(randint)
 del ydata[0]
 line.set_xdata(np.arange(len(ydata)))
 line.set_ydata(ydata) # update data
 plt.draw() # update plot

I get a plot window that pops up, but no data appears and nothing gets redrawn...any idea what I'm doing wrong?

• Stefan Bollmann: On Mac 10.6.8 with python3.3 and python2.7, matplotlib 1.3.0, numpy 1.7.1 everything is fine. Do you have the latest packages? What happens if you use another backend - if I insert import matplotlib; matplotlib.use('Qt4Agg') then there is nothing redrawn, too. You might need another backend?!
• tacaswell: What does matplotlib.get_backend() return? Try adding a plt.pause(.1) in your loop.
• lollercoaster: my backend is 'WXAgg - I don't know if that's good or bad. and the pause seems to work!
• tacaswell: possible duplicate of make matplotlib draw() only show new point

Answer 1

The issue you are having is due to the way that gui mainloops work. When ever you plot call draw events get added to the queue of events for the main loop to process. If you add them as fast as possible the loop can never clear it's queue and actually draw to the screen.

Adding a plt.pause(.1) will pause the loop and allow the main loop (at the risk of being anthropomorphic) 'catch it's breath' and update the widgets on the screen

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