title: errorbar, but not line, as marker symbol in python matplotlib legend
body: <p>I have a errorbar plot with only one data point (i.e. one errorbar) per data set. Therefore I would like to have a single errorbar symbol in the legend as well.
  The single one can be achieved by <code>legend(numpoints=1)</code>. Using this in the following code:</p>
  
  <pre><code>    import matplotlib.pyplot as plt
  
      fig, ax = plt.subplots()
  
      ax.errorbar(x=[0.3], y=[0.7], xerr=[0.2], marker='+', markersize=10, label='horizontal marker line')
      ax.errorbar(x=[0.7], y=[0.3], yerr=[0.2], marker='+', markersize=10, label='is too long')
  
      ax.set_xlim([0,1])
      ax.set_ylim([0,1])
      ax.legend(numpoints=1) # I want only one symbol
  
      plt.show()
  </code></pre>
  
  <p>results in these symbols in the legend:
  <a href="https://i.stack.imgur.com/5E2PO.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5E2PO.jpg" alt="errorbars in legend are mixed up with lines"></a></p>
  
  <p>As you see, the errorbars are mixed up with horizontal lines, that make sense when there are more than one error bars to be connected (using  <code>legend(numpoints=2)</code> or higher), but look ugly in my case.</p>
  
  <p>How can I get rid of the lines in the legend markers without loosing the errorbars?</p>