update figure inside loop

Question

I'm new to Python matplotlib and want to make a figure updated in a loop. This is some simple test code. I want to draw 3 graphs, each for 2 seconds, with different slopes each time. Of course, I know I should read some more about matplotlib but I need to do something quick. What is wrong below? Thanks!

#!/usr/bin/env python

import matplotlib.pyplot as plt
import numpy as np
import time

def demo(a):
    y = [xt*a+1 for xt in x]
    ax.plot(x,y)

if __name__ == '__main__':
    plt.ion()
    fig, ax = plt.subplots()
    ax.set_ylim([0,15])
    x = range(0,5)
    for a in range(1,4):
        demo(a)
        time.sleep(2)

I thought plt.ion() makes it interactive and ax.plot(x,y) take effect instantly (but guess not). I tried adding ax.draw() after ax.plot(x,y) but it requires some arguments like artist or something which I don't know yet :). Also, I have this error message coming (Ubuntu 14.04.LTS).

Xlib:  extension "XInputExtension" missing on display ":1.0".
X Error: BadDrawable (invalid Pixmap or Window parameter) 9
  Major opcode: 62 (X_CopyArea)
  Resource id:  0x0

EDIT : I add the correct code below. (according to Estilus's solution to which I modified a little for correct graph display.

#!/usr/bin/env python

import matplotlib.pyplot as plt
import numpy as np
import time

def demo(a):
    plt.cla()
    y = [xt*a+1 for xt in x]
    ax.set_ylim([0,15])
    ax.plot(x,y)

if __name__ == '__main__':
    plt.ion()
    fig, ax = plt.subplots()
    x = range(5)
    for a in range(1,4):
        demo(a)
        plt.pause(3)
        plt.draw()

Solution

So plt.ion() needs to be paused for a short period of time for it to be interactive. Otherwise, you'll just run into a frozen white screen. Secondly you want to use draw() to update the figure. Therefore, your code would look something like this:

import matplotlib.pyplot as plt
import numpy as np
import time

def demo(a):
    y = [xt*a+1 for xt in x]
    ax.plot(x,y)

if __name__ == '__main__':
    plt.ion()
    fig, ax = plt.subplots()
    ax.set_ylim([0,15])
    x = range(0,5)
    for a in range(1,4):
        demo(a)
        plt.pause(3)
        plt.draw()

I set the pause time to be 3 seconds, but it could be a pretty small number if you'd like (like 0.000001). Hope this helped!

Comments

[Chan Kim]

I did exactly as you showed me but it gives me this error. (maybe not python related). and I see no figures excepts a short flashing figure window after a couple of seconds. Xlib: extension "XInputExtension" missing on display ":1.0". X Error: BadDrawable (invalid Pixmap or Window parameter) 9 Major opcode: 62 (X_CopyArea) Resource id: 0x0

[Frodon]

@ChanKim this is not Python related. It's a problem with your graphic environment (virtual machine ?)

[Chan Kim]

I know, so I copied your code to my CentOS6.7 machine, but no picture window shows up. The script just ends after some seconds. What could be wrong? Are you sure your modified code runs ok in your machine?

[Chan Kim]

Hi, this works. except I moved ax.set_ylim([0,15]) to inside demo(). otherwise the ylim is not correctly set. Thanks!