title: Adding an edge to an X marker in a matplotlib scatterplot
body: <p>I'm making a matplotlib plot in python. I have one marker that I am 
  putting onto the plot using: </p>
  
  <pre><code>plt.scatter(x_position,y_position,c=z_position,cmap=cm.bwr,marker='x',s=500,linewidth=4)
  </code></pre>
  
  <p>As you can see, the color of the marker is a reflection of the z position.</p>
  
  <p>My only problem is that sometimes the 'X' is not entirely distinct from the background (depending on the color of course), so I wanted to put a black outline around the 'X' marker.</p>
  
  <p>I tried changing edgecolor, but this didn't seem to have an effect. I think this is because I have made the marker an 'X' and so there is no edge in the same way a regular circle scatter plot would have an edge?</p>
  
  <p>Thanks. </p>