[Matplotlib-users] odd behavior with 'nearest' interpolation

[Smit, Christine E. (GSFC-610.2)[TELOPHASE CORP]]

Hi! I am using matplotlib v 1.4.3 with Python 2.7.10 :: Anaconda 2.4.0 (64-bit).

What I am doing here is creating a 7x7 pixel image from a 7x7 matrix. I expect to see one pixel per data point, but that's not what I'm seeing. Instead of a diagonal make up of single pixels, I get an odd 2x2 pixel blob in the middle of the correct one pixel diagonal.

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import numpy as np
import matplotlib.pylab as plt


if __name__ == "__main__":
    n = 7
    data = np.identity(n, float)

    # Create an nxn size figure with no frame
    fig = plt.figure(figsize=(n, n), frameon=False)

    # make the axes to the edge of the figure
    ax = plt.Axes(fig, [0.0, 0.0, 1.0, 1.0])
    # turn the axes off
    ax.set_axis_off()
    # add the axes to this figure
    fig.add_axes(ax)
    # show the data. Don't do any interpolation.
    ax.imshow(data, interpolation='nearest', origin='lower',aspect='auto')
    # Save the figure at 1 dot per inch, which should mean 1 data point per
    # pixel
    fig.savefig("image.png", dpi=1)

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Since I'm not sure that if I can attach the png image I get, here is a ppm version of the image I get (between the ------). Save this image.ppm minus the dashes and you should be able to open it in something like gimp.

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P3
# CREATOR: GIMP PNM Filter Version 1.1
7 7
255
0
0
127
0
0
127
0
0
127
0
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127
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0
127
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127
127
0
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127
---------------------------------------------------------------------------

Thanks.
Christine
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