A imshow map without fill color

Dear all, I would like to create imshow map using ax.imshow(arr). However, I
got the image as Fig. 1. Is there any setting to get the image as Fig. 2
(i.e., without fill color for the cell).

<http://matplotlib.1069221.n5.nabble.com/file/n43975/1.png>
Fig. 1

<http://matplotlib.1069221.n5.nabble.com/file/n43975/2.jpg>
Fig. 2

Thanks in advance for your answer.
Jiacong Huang

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