title: Error in (some) calls of matplotlib.pyplot.tricontourf: TypeError: Last contourf arg must give levels; see help(contourf)
body: <p>I am writing some code in which I want to make a set of plots. 
  For this I am using the tricontourf function of matplotlib (v 2.2.3), since the data is unstructured. Every array which I want to plot contains 1000 float32 values. </p>
  
  <p>In some cases everything works fine and the plots come out as expected. However for some of these arrays I get the following Error message:</p>
  
  <blockquote>
    <p>TypeError: Last contourf arg must give levels; see help(contourf)
    Exception RuntimeError: RuntimeError('cannot join current thread',) in  ignored</p>
  </blockquote>
  
  <p>The error only appears when I give the contour levels as an integer. If I instead specify the "levels" arg as an np.linspace with the same number of levels, I get no problem.
  But as I have no idea why the error occurs in first place (since for some cases it works), I am kind of hesitated to just replace the argument with that.</p>
  
  <p>Hopefully anyone has experience with that and an answer to my question.</p>
  
  <p>The corresponding line of code in my program is: </p>
  
  <pre class="lang-py prettyprint-override"><code>    import numpy as np
      import matplotlib.pyplot as plt
      plt.rcParams.update({'font.size': 22})
  
  
  
      x = 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 
      y = 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 
      z1 = 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 
      z2 = 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 
  
      def plot_function1(x, y, z):
          print('plotting evil plans')
          fig = plt.figure(figsize=(36, 32))
          ax = fig.add_subplot(111)
          ax.tricontourf(x, y, z, np.linspace(min(z), max(z), 1000), cmap='magma', antialiased=False)
          plt.close(fig)
  
      def plot_function2(x, y, z):
          print('plotting')
          fig = plt.figure(figsize=(36, 32))
          ax = fig.add_subplot(111)
          ax.tricontourf(x, y, z, 1000, cmap='magma', antialiased=False)
          plt.close(fig)
  
      x, y, z1, z2 = np.asarray(x), np.asarray(y), np.asarray(z1), np.asarray(z2)
  
      print('shape of x1 = {} and dtype = {}'.format(np.shape(x), x.dtype))
      print('shape of y1 = {} and dtype = {}'.format(np.shape(y), y.dtype))
      print('shape of z1 = {} and dtype = {}'.format(np.shape(z1), z1.dtype))
      print('shape of z2 = {} and dtype = {}'.format(np.shape(z2), z2.dtype))
  
      plot_function1(x, y, z1)
      plot_function2(x, y, z1)
      plot_function1(x, y, z2)
      plot_function2(x, y, z2)
  </code></pre>
  
  <p>The above example works fine if the value at positions x and y is given by z1, but fail to plot with the 2nd version of the plot function, i.e. the one with an integer as "levels" argument.</p>
  
  <p>The complete output of the above code is:</p>
  
  <blockquote>
    <p>shape of x1 = (1000,) and dtype = float64<br>
    shape of y1 = (1000,) and dtype = float64<br>
    shape of z1 = (1000,) and dtype = float64<br>
    shape of z2 = (1000,) and dtype = float64<br>
    plotting evil plans<br>
    plotting<br>
    plotting evil plans<br>
    plotting<br>
    Traceback (most recent call last):<br>
      File "stack_example.py", line 40, in <br>
        plot_function2(x, y, z2)<br>
      File "stack_example.py", line 27, in plot_function2<br>
        ax.tricontourf(x, y, z, 1000, cmap='magma', antialiased=False)<br>
      File "/home/patrik/.local/lib/python2.7/site-packages/matplotlib/axes/_axes.py", line 8144, in tricontourf<br>
        return mtri.tricontourf(self, *args, **kwargs)<br>
      File "/home/patrik/.local/lib/python2.7/site-packages/matplotlib/tri/tricontour.py", line 282, in tricontourf<br>
        return TriContourSet(ax, *args, **kwargs)<br>
      File "/home/patrik/.local/lib/python2.7/site-packages/matplotlib/tri/tricontour.py", line 40, in __init__<br>
        ContourSet.<strong>init</strong>(self, ax, *args, **kwargs)<br>
      File "/home/patrik/.local/lib/python2.7/site-packages/matplotlib/contour.py", line 890, in __init__<br>
        kwargs = self._process_args(*args, **kwargs)<br>
      File "/home/patrik/.local/lib/python2.7/site-packages/matplotlib/tri/tricontour.py", line 51, in _process_args<br>
        tri, z = self._contour_args(args, kwargs)<br>
      File "/home/patrik/.local/lib/python2.7/site-packages/matplotlib/tri/tricontour.py", line 94, in _contour_args<br>
        self._contour_level_args(z, args[1:])<br>
      File "/home/patrik/.local/lib/python2.7/site-packages/matplotlib/contour.py", line 1220, in _contour_level_args<br>
        .format(fn))<br>
    TypeError: Last contourf arg must give levels; see help(contourf)</p>
  </blockquote>