Matplotlib's tricontour interesting behaviour

Matplotlib was happy to plot contours of triangular meshes in the past. Though they were not actually triangular, they were triangulated (by OpenFOAM's sample utility) rectangular meshes. Recently I've encountered rather strange thing, all my attempts to plot isoline of a value on a triangular mesh which initially wasn't rec

	data = N.loadtxt(filename)
	x = 100*data[:, xidx]
	y = 100*data[:, yidx]
	fl = data[:, flidx]
	asp = 4.5/(xlim[1] - xlim[0])
	P.figure(figsize=(8, 8*asp))
	P.clf()
	P.tricontour(x, y, fl, [0.5], colors='black')

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ended up with

	Traceback (most recent call last):
	File "./plot.py", line 116, in <module>
	plot_planes(s)
	File "./plot.py", line 75, in plot_planes
	P.tricontour(x, y, fl, [0.5], colors='black')
	File "/usr/lib/pymodules/python2.7/matplotlib/pyplot.py", line 3090, in tricontour
	ret = ax.tricontour(*args, **kwargs)
	File "/usr/lib/pymodules/python2.7/matplotlib/axes.py", line 8922, in tricontour
	return mtri.tricontour(self, *args, **kwargs)
	File "/usr/lib/pymodules/python2.7/matplotlib/tri/tricontour.py", line 280, in tricontour
	return TriContourSet(ax, *args, **kwargs)
	File "/usr/lib/pymodules/python2.7/matplotlib/tri/tricontour.py", line 36, in __init__
	ContourSet.__init__(self, ax, *args, **kwargs)
	File "/usr/lib/pymodules/python2.7/matplotlib/contour.py", line 780, in __init__
	self._process_args(*args, **kwargs)
	File "/usr/lib/pymodules/python2.7/matplotlib/tri/tricontour.py", line 47, in _process_args
	tri, z = self._contour_args(args, kwargs)
	File "/usr/lib/pymodules/python2.7/matplotlib/tri/tricontour.py", line 82, in _contour_args
	Triangulation.get_from_args_and_kwargs(*args, **kwargs)
	File "/usr/lib/pymodules/python2.7/matplotlib/tri/triangulation.py", line 172, in get_from_args_and_kwargs
	triangulation = Triangulation(x, y, triangles, mask)
	File "/usr/lib/pymodules/python2.7/matplotlib/tri/triangulation.py", line 72, in __init__
	dt = delaunay.Triangulation(self.x, self.y)
	File "/usr/lib/pymodules/python2.7/matplotlib/delaunay/triangulate.py", line 123, in __init__
	self.hull = self._compute_convex_hull()
	File "/usr/lib/pymodules/python2.7/matplotlib/delaunay/triangulate.py", line 158, in _compute_convex_hull
	hull.append(edges.pop(hull[-1]))
	KeyError: 5685

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while if I try to use scipy triangulation

	from scipy.spatial import Delaunay
	from matplotlib.tri import Triangulation
	data = N.loadtxt(filename)
	x = 100*data[:, xidx]
	y = 100*data[:, yidx]
	pts = N.column_stack((x, y))
	tri = Delaunay(pts)
	mtri = Triangulation(x, y, tri.simplices.copy())
	fl = data[:, flidx]
	asp = 4.5/(xlim[1] - xlim[0])
	P.figure(figsize=(8, 8*asp))
	P.clf()
	P.tricontour(mtri, fl, [0.5], colors='black')

view raw gistfile1.py hosted with ❤ by GitHub

everything goes more or less OK.

Thought matplotlib and scipy use the same algorithm for triangulation appears I was wrong.