contour and contourf produce conflicing results for certain projections

还没有人认领这个 Issue。

Originally posted [here](http://stackoverflow.com/questions/33850052/matplotlib-why-inconsistent-results-between-contour-and-contourf) because I wasn't convinced it was a bug (I'm new to matplotlib)... but I'm increasingly convinced it is a bug, so here we are.

I'm plotting some NCEP temperature anomaly data using the stereographic projection. When I plot the data with filled contours, contourf, I see the first picture below. Note the red (positive values) over most of North America. When I change contourf to contour, making no other changes, I see the second picture below. Note the streak of blue (negative values) over central North America.
The contour result with the blue is what is actually in the data, and therefore what I expect to see. The contourf result is not what I expect. Is this a bug, or am I using these tools improperly? (Update: The first plot above should look like the below plot, which was produced using the cylindrical projection:)
The python code is reproduced below... you can obtain the data as a small binary file [here](https://onedrive.live.com/redir?resid=9F18EDDCF4E99E72!108&authkey=!ADXWQucUMyXgkJk&ithint=file%2cbin) if you want to try it yourself.

Some more notes... (1) shifting the longitude range to -180 to 180 using `addcyclic` and `shiftgrid` does not help. (2) the Lambert Conformal ('lcc') projection produces results that are _less_ wrong, but still not correct, when using contourf. (3) the Alberts Equal Area ('aea') projection centered on North America looks correct, however when I use the North Pole-centric version ('nplaea') I only see correct results for certain values of `lon_0`. I am happy to provide an example if this turns out to be a different issue.

```
import numpy as np
import matplotlib.pyplot as plt

# read data

f = open('data5.bin', 'r')
lat = np.fromfile(f,dtype=np.float32,count=73)
lon = np.fromfile(f,dtype=np.float32,count=144)
data = np.reshape(np.fromfile(f,dtype=np.float32,count=-1),(73,144))
f.close()

# plot

m = Basemap(width=10000000,height=6000000,
resolution='l',projection='stere',\
lat_ts=50,lat_0=50,lon_0=253)
m.drawcoastlines()
lon2d, lat2d = np.meshgrid(lon,lat)
x, y = m(lon2d,lat2d)
mymap = plt.contourf(x,y,data,levels=np.arange(17)-8,cmap=plt.cm.bwr)
plt.colorbar(mymap,orientation='vertical',shrink=0.75)
plt.show()
```

首先使用 data5.bin 运行复现的 Python 示例，并比较立体投影下的 contourf 和 contour。检查投影后的 x/y 网格如何传递给每个绘图调用，然后比较 issue 中描述的圆柱投影、Lambert Conformal 和 Albers 结果。当 contourf 与预期数据一致（包括北美区域），且不更改输入数据时，即视为完成。

由索引模型根据 Issue 内容生成。