contourf on cylindrical projection with irregular latlon grid (tri=True) is calling shiftdata when it shouldn't

lib/mpl_toolkits/basemap/init.py から始め、contourf ラッパーをたどって shiftdata と traceback に示されている itemindex チェックを確認します。円筒図法、不規則な 1D 配列、tri=True、latlon=True を使って例を再現し、その後、正射図法の場合と比較します。完了条件は、円筒図法の場合に ambiguous-array の ValueError が発生しなくなり、規則格子の動作が維持されることです。

I ran into this problem while trying to plot interpolated data NOT on a regular latlon grid. Orthographic projection works but cylindrical does not.

Relevant code:

fig = plt.figure()
ax = fig.add_subplot(111)
levels = 20
m = Basemap(projection="cyl")  # causes ValueError
# m = Basemap(projection="ortho", lat_0=0., lon_0=0., resolution='l')  # works, does not cause ValueError
MC = m.contourf(lons_deg, lats_deg, vals, levels, ax=ax, tri=True, latlon=True)  # latlon=True interprets first two args as lon and lat respectively

Note that lats_deg, lons_deg, and vals are 1d np arrays with irregular point spacing (no meshgrid). I used tri=True to allow interpolation. This works fine on orthographic projection. But with cylindrical I get the following error:

Traceback (most recent call last):
  File "IcosahedralGeodesicLattice.py", line 146, in <module>
    test_lattice.plot_data(data)
  File "/home/wesley/programming/Mapping/Lattice.py", line 86, in plot_data
    MC = m.contourf(lons_deg, lats_deg, vals, levels, ax=ax, tri=True, latlon=True)  # latlon=True interprets first two args as LON and LAT RESPECTIVELY
  File "/home/wesley/.local/lib/python3.5/site-packages/mpl_toolkits/basemap/__init__.py", line 544, in with_transform
    fix_wrap_around=plotfunc.__name__ not in ["scatter"])
  File "/home/wesley/.local/lib/python3.5/site-packages/mpl_toolkits/basemap/__init__.py", line 4797, in shiftdata
    if fix_wrap_around and itemindex:
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

Looking at the relevant file, the docstring for shiftdata says "Only valid for cylindrical/pseudo-cylindrical global projections and data on regular lat/lon grids. longitudes and data can be 1-d or 2-d, if 2-d it is assumed longitudes are 2nd (rightmost) dimension."

So should it not be being called with tri=True? Sorry if I have misunderstood something, let me know. Thanks!