title: Contour line error with plt.contour in python 3
body: <p>I am plotting a contour plot in python 3 with matplotlib, and I am getting a strange result. At first, I was using plt.contourf, and notices there was a strange north-south linear artifact in the data that I knew shouldn't be there (I used simulated data). So I changed plt.contourf to plt.contour, and the problem seems to be that some of the edge contours are deformed for some reason (see picture).</p>
  <p><a href="https://i.stack.imgur.com/A4w2I.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/A4w2I.png" alt="enter image description here" /></a></p>
  <p>Unfortunately, it is hard for me to past a simple version of my code because this is part of a large GUI based app. Here is what I am doing though.</p>
  <pre><code>#grid the x,y,z data so it can be used in the contouring
  self.beta_zi = 
  #This is matplot griddata, not the scipy.interpolate.griddata
  griddata(self.output_df['x'].values,self.output_df['y'].values,
                                self.output_df['Beta'].values,
                                self.cont_grid_x,
                                self.cont_grid_y,
                                interp='linear')
  
      #call to the contour itself
  self.beta_contour=self.beta_cont_ax.contour(self.cont_grid_x,self.cont_grid_y,
                                    self.beta_zi,
                                    levels=np.linspace(start=0,stop=1, num=11, endpoint=True),
                                    cmap=cm.get_cmap(self.user_beta_cmap.get()))        
  </code></pre>
  <p>This seems like a simple problem based on the edges. Has anyone seen this before that can help. I am use a TK backend, which works better with the tkinter based GUI I wrote.</p>
  <p>UPDATE: I also tried changing to scipy.interpolate.griddata because matplot's griddata is deprecated, but the problem is the same and persists, so it must be with the actual contour plotting function.</p>