error using pcolor from matplotlib

Question

Im new on python. Im trying to use pcolor from matplotlib for gridx, gridy and V, all size 401x121. Function is as following:

def plotSpatialKM(V,gridx,gridy,step_h):
    plt.figure(figsize=(18,8), dpi=80, facecolor='white')
    plt.pcolor(gridx,gridy,V)
    plt.colorbar()
    plt.xlim(gridx.min(), gridx.max())
    plt.ylim(gridy.min(), gridy.max())
    plt.xlabel('x/h',fontsize=FONTSIZE)
    plt.ylabel('y/h',fontsize=FONTSIZE)
    plt.xticks(fontsize=FONTSIZE)
    plt.yticks(fontsize=FONTSIZE)    
    plt.show()
    return(1)

But all the time, it causes following error:

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Python27\lib\lib-tk\Tkinter.py", line 1410, in __call__
    return self.func(*args)
  File "C:\Python27\lib\site-packages\matplotlib\backends\backend_tkagg.py", line 276, in resize
    self.show()
  File "C:\Python27\lib\site-packages\matplotlib\backends\backend_tkagg.py", line 348, in draw
    FigureCanvasAgg.draw(self)
  File "C:\Python27\lib\site-packages\matplotlib\backends\backend_agg.py", line 439, in draw
    self.figure.draw(self.renderer)
  File "C:\Python27\lib\site-packages\matplotlib\artist.py", line 54, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "C:\Python27\lib\site-packages\matplotlib\figure.py", line 999, in draw
    func(*args)
  File "C:\Python27\lib\site-packages\matplotlib\artist.py", line 54, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "C:\Python27\lib\site-packages\matplotlib\axes.py", line 2086, in draw
    a.draw(renderer)
  File "C:\Python27\lib\site-packages\matplotlib\artist.py", line 54, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "C:\Python27\lib\site-packages\matplotlib\collections.py", line 755, in draw
    return Collection.draw(self, renderer)
  File "C:\Python27\lib\site-packages\matplotlib\artist.py", line 54, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "C:\Python27\lib\site-packages\matplotlib\collections.py", line 244, in draw
    self.update_scalarmappable()
  File "C:\Python27\lib\site-packages\matplotlib\collections.py", line 609, in update_scalarmappable
    raise ValueError('Collections can only map rank 1 arrays')
ValueError: Collections can only map rank 1 arrays

Please help out!

Please post a self-contained example, which runs out of the box. My guess is that your input arrays have the wrong shape, but I can't tell from the code you're posting. — David Zwicker
– David Zwicker, Commented Feb 7, 2013 at 9:59
well, I checked the size of 3 arrays, perfectly match. Sorry I cannot put all the code, since data file is so big! — nguyen van linh
– nguyen van linh, Commented Feb 7, 2013 at 10:21
You should still give a minimal example with random data, where the error occurs, such that people can actually reproduce the problem. You don't need to give your original data. Anyway, what are the dimensions of your input arrays? — David Zwicker
– David Zwicker, Commented Feb 7, 2013 at 10:35
as I meantion, all 3 vectors are 401x121. Gridx like [[-0.006 -0.0059 -0.0058...] [-0.006 -0.0059 -0.0058...] .........................]] Gridy like [[0 0 0 0 0..............] [0.001 0.0001 ...........] .........................]] and V is same size, but magnitude! Thanks for your help! — nguyen van linh
– nguyen van linh, Commented Feb 8, 2013 at 1:07

paugier · Accepted Answer · 2014-07-03 08:17:58Z

1

I have encountered the same error using np.matrix.

import matplotlib.pyplot as plt
import numpy as np

A = np.random.random([8, 8])

plt.colormesh(A) # no error
M = np.matrix(A)
plt.colormesh(M) # the same error

The numpy matrices do not act at all like ndarrays (for example all(M[0][0][0][0][0] == M[0]) is True even if M.ndim == 2) and here matplotlib does not take that into account... You just have to create a ndarray from the matrix:

plt.colormesh(np.array(M)) # no error

answered Jul 3, 2014 at 8:17

paugier

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