<p dir="auto">As reported to matplotlib.users, passing duplicate points to tricontour can cause it to hang.  Here is a minimal example:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="import matplotlib.pyplot as plt
import matplotlib.tri as tri
import numpy as np
from numpy.random import uniform, seed

seed(0)
npts = 7
x = uniform(-2,2,npts)
y = uniform(-2,2,npts)
z = x*np.exp(-x**2-y**2)

y[1:2] = y[0]  # Duplicate points make tricontour hang!
x[1:2] = x[0]
triang = tri.Triangulation(x, y)
plt.triplot(triang)
plt.tricontour(triang, z, 15, linewidths=0.5, colors='k')
plt.show()"><pre class="notranslate"><code class="notranslate">import matplotlib.pyplot as plt
import matplotlib.tri as tri
import numpy as np
from numpy.random import uniform, seed

seed(0)
npts = 7
x = uniform(-2,2,npts)
y = uniform(-2,2,npts)
z = x*np.exp(-x**2-y**2)

y[1:2] = y[0]  # Duplicate points make tricontour hang!
x[1:2] = x[0]
triang = tri.Triangulation(x, y)
plt.triplot(triang)
plt.tricontour(triang, z, 15, linewidths=0.5, colors='k')
plt.show()
</code></pre></div>
<p dir="auto">Plotting with markeredgecolor="none" causes markers to not display (probably fully transparent) in current master 1.5.x. It might be related to <a href="https://github.com/matplotlib/matplotlib/pull/598" data-hovercard-type="pull_request" data-hovercard-url="/matplotlib/matplotlib/pull/598/hovercard">this pull request</a>. The plot in Out[3] below does not show anything. In matplotlib 1.4 it displays markers as expected.</p>
<div class="highlight highlight-source-python notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="~/matplotlib (master)$ ipython
Python 2.7.6 (default, Aug 25 2014, 18:32:19) 
Type &quot;copyright&quot;, &quot;credits&quot; or &quot;license&quot; for more information.

IPython 2.2.0 -- An enhanced Interactive Python.
?         -&gt; Introduction and overview of IPython's features.
%quickref -&gt; Quick reference.
help      -&gt; Python's own help system.
object?   -&gt; Details about 'object', use 'object??' for extra details.

In [1]: %matplotlib
Using matplotlib backend: TkAgg

In [2]: import matplotlib; import matplotlib.pyplot as plt; import numpy as np

In [3]: plt.plot(np.arange(3), 'ro', mec='none')
Out[3]: [&lt;matplotlib.lines.Line2D at 0x4171610&gt;] 

In [4]: matplotlib
Out[4]: &lt;module 'matplotlib' from '/apps/stonefs1/azhu/venv/el5-stonelib/lib/python2.7/site-packages/matplotlib-1.5.x-py2.7-linux-x86_64.egg/matplotlib/__init__.pyc'&gt;

In [5]: matplotlib.__version__
Out[5]: '1.5.x'"><pre class="notranslate"><span class="pl-c1">~</span><span class="pl-c1">/</span><span class="pl-en">matplotlib</span> (<span class="pl-s1">master</span>)$ <span class="pl-s1">ipython</span>
<span class="pl-v">Python</span> <span class="pl-c1">2.7</span><span class="pl-c1">.6</span> (<span class="pl-s1">default</span>, <span class="pl-v">Aug</span> <span class="pl-c1">25</span> <span class="pl-c1">2014</span>, <span class="pl-c1">18</span>:<span class="pl-c1">32</span>:<span class="pl-c1">19</span>) 
<span class="pl-v">Type</span> <span class="pl-s">"copyright"</span>, <span class="pl-s">"credits"</span> <span class="pl-c1">or</span> <span class="pl-s">"license"</span> <span class="pl-k">for</span> <span class="pl-s1">more</span> <span class="pl-s1">information</span>.

<span class="pl-v">IPython</span> <span class="pl-c1">2.2</span><span class="pl-c1">.0</span> <span class="pl-c1">-</span><span class="pl-c1">-</span> <span class="pl-v">An</span> <span class="pl-s1">enhanced</span> <span class="pl-v">Interactive</span> <span class="pl-v">Python</span>.
?         <span class="pl-c1">-&gt;</span> <span class="pl-v">Introduction</span> <span class="pl-c1">and</span> <span class="pl-s1">overview</span> <span class="pl-s1">of</span> <span class="pl-v">IPython</span>'<span class="pl-s1">s</span> <span class="pl-s1">features</span>.
<span class="pl-c1">%</span><span class="pl-s1">quickref</span> <span class="pl-c1">-</span><span class="pl-c1">&gt;</span> <span class="pl-v">Quick</span> <span class="pl-s1">reference</span>.
<span class="pl-s1">help</span>      <span class="pl-c1">-</span><span class="pl-c1">&gt;</span> <span class="pl-v">Python</span>'<span class="pl-s1">s</span> <span class="pl-s1">own</span> <span class="pl-s1">help</span> <span class="pl-s1">system</span>.
<span class="pl-s1">object</span>?   <span class="pl-c1">-&gt;</span> <span class="pl-v">Details</span> <span class="pl-s1">about</span> <span class="pl-s">'object'</span>, <span class="pl-s1">use</span> <span class="pl-s">'object??'</span> <span class="pl-k">for</span> <span class="pl-s1">extra</span> <span class="pl-s1">details</span>.

<span class="pl-v">In</span> [<span class="pl-c1">1</span>]: <span class="pl-c1">%</span><span class="pl-s1">matplotlib</span>
<span class="pl-v">Using</span> <span class="pl-s1">matplotlib</span> <span class="pl-s1">backend</span>: <span class="pl-v">TkAgg</span>

<span class="pl-v">In</span> [<span class="pl-c1">2</span>]: <span class="pl-k">import</span> <span class="pl-s1">matplotlib</span>; <span class="pl-k">import</span> <span class="pl-s1">matplotlib</span>.<span class="pl-s1">pyplot</span> <span class="pl-k">as</span> <span class="pl-s1">plt</span>; <span class="pl-k">import</span> <span class="pl-s1">numpy</span> <span class="pl-k">as</span> <span class="pl-s1">np</span>

<span class="pl-v">In</span> [<span class="pl-c1">3</span>]: <span class="pl-s1">plt</span>.<span class="pl-en">plot</span>(<span class="pl-s1">np</span>.<span class="pl-en">arange</span>(<span class="pl-c1">3</span>), <span class="pl-s">'ro'</span>, <span class="pl-s1">mec</span><span class="pl-c1">=</span><span class="pl-s">'none'</span>)
<span class="pl-v">Out</span>[<span class="pl-c1">3</span>]: [<span class="pl-c1">&lt;</span><span class="pl-s1">matplotlib</span>.<span class="pl-s1">lines</span>.<span class="pl-v">Line2D</span> <span class="pl-s1">at</span> <span class="pl-c1">0x4171610</span><span class="pl-c1">&gt;</span>] 

<span class="pl-v">In</span> [<span class="pl-c1">4</span>]: <span class="pl-s1">matplotlib</span>
<span class="pl-v">Out</span>[<span class="pl-c1">4</span>]: <span class="pl-c1">&lt;</span><span class="pl-s1">module</span> <span class="pl-s">'matplotlib'</span> <span class="pl-s1">from</span> <span class="pl-s">'/apps/stonefs1/azhu/venv/el5-stonelib/lib/python2.7/site-packages/matplotlib-1.5.x-py2.7-linux-x86_64.egg/matplotlib/__init__.pyc'</span><span class="pl-c1">&gt;</span>

<span class="pl-v">In</span> [<span class="pl-c1">5</span>]: <span class="pl-s1">matplotlib</span>.<span class="pl-s1">__version__</span>
<span class="pl-v">Out</span>[<span class="pl-c1">5</span>]: <span class="pl-s">'1.5.x'</span></pre></div>