contour and contourf produce conflicing results for certain projections

まず、data5.bin を使って再現された Python の例を実行し、ステレオ投影で contourf と contour を比較します。投影後の x/y グリッドが各プロット呼び出しにどのように渡されているかを確認し、次に issue に記載されている円筒図法、Lambert Conformal、Albers の結果を比較します。入力データを変更せずに、北米地域を含めて contourf が期待されるデータと一致すれば完了です。

Originally posted here 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 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()