Matplotlib: no effect of set_data in imshow for the plot

Question

I have a strange error which I can't fix without your help. After I set an image with imshow in matplotlib it stays the same all the time even if I change it with the method set_data . Just take a look on this example:

import numpy as np
from matplotlib import pyplot as plt


def newevent(event):
    haha[1,1] += 1
    img.set_data(haha)
    print img.get_array()    # the data is change at this point
    plt.draw()

haha = np.zeros((2,2))
img = plt.imshow(haha)
print img.get_array()        # [[0,0],[0,0]]
plt.connect('button_press_event', newevent)
plt.show()

After I plot it, the method set_data doesn't change anything inside the plot. Can someone explain me why?

EDIT

Just added a few lines to point out what I actually want to do. I want to redraw the data after I press a mouse button. I don't want to delete the whole figure, because it would be stupid if only one thing changes.

Answer 1

The problem is because you have not updated the pixel scaling after the first call.

When you instantiate imshow , it sets vmin and vmax from the initial data, and never touches it again. In your code, it sets both vmin and vmax to 0, since your data, haha = zeros((2,2)) , is zero everywhere.

Your new event should include autoscaling with img.autoscale() , or explicitly set new scaling terms by setting img.norm.vmin/vmax to something you prefer.

The function to set the new vmin and vmax is:

img.set_clim(vmin=new_vim, vmax=new_vmax)

Answer 2

Does this give you the output you expect?

import numpy as np
from matplotlib import pyplot as plt

haha = np.zeros((2,2))
img = plt.imshow(haha)
print img.get_array()        # [[0,0],[0,0]]
haha[1,1] += 1

img.set_data(haha)    
img = plt.imshow(haha)       # <<------- added this line  
print img.get_array()        # [[0,0],[0,1]]
plt.show()

When I display the plot twice (once before the change to haha , and at the end), it does change.