Document idiosyncratic behavior of `transform` kwarg

Bug report

Bug summary

Passing transform=None through imshow results in a blank plot.

I've been playing around a little bit with data transforms and projections in matplotlib. For one of the things I've been testing out, I also want to see how my data looks if the transform isn't set by the user, so I passed in transform=None as a kwarg to imshow. This resulted in a blank plot. I'm not sure if this is a preferred outcome, but I personally would have expected a plot without a transform set, but the data showing.

Code for reproduction

from numpy import random
import matplotlib.pyplot as plt

data = random.random((20,20))

fig, (ax1, ax2)  = plt.subplots(2)

ax1.imshow(data)
ax2.imshow(data, transform=None)
plt.show()

Outcome

Matplotlib version

• Operating system: OSX Sierra 10.12.6
• Matplotlib version: 2.2.2 and 3.0.0
• Matplotlib backend (print(matplotlib.get_backend())): MacOSX
• Python version: 3.6.5 and 3.6.6

I tried testing this script out in two different conda environments that I have on my machine with the same result. Both envs I tested this script out on use matplotlib pulled from conda-forge. One environment has v2.2.2 and the other has v3.0.0

首先运行提供的 imshow 复现，并阅读 imshow 入口点及其对 transform 的处理。确定 transform=None 的预期行为，然后在相关的 Matplotlib 文档中记录该行为及其后果；当文档中描述的行为与已确认的结果一致时，即表示完成。