title: Horizontal line to infinity on one side only in matplotlib
body: <p>I'd like to plot a line that goes to infinity, but starting from a finite point. For simplicity, let's say that the line can be horizontal. I would like to plot a line from <code>(0, 0)</code> to <code>(inf, 0)</code>.</p>
  <p>Using <code>hlines</code>:</p>
  <pre><code>&gt;&gt;&gt; fig, ax = plt.subplots()
  &gt;&gt;&gt; ax.hlines(0, 0, np.inf)
  .../python3.8/site-packages/matplotlib/axes/_base.py:2480: UserWarning: Warning: converting a masked element to nan.
    xys = np.asarray(xys)
  </code></pre>
  <p>The result is an empty plot.</p>
  <p><code>axhline</code> has a starting parameter, but it is in axis coordinates rather than data. Similar problem for <code>axline</code>. Is there a way to plot a (horizontal) line with one end in data coordinates and the other at infinity?</p>
  <p>The motivation behind this is that I'd like to be able to plot some cumulative probabilities without setting data past the last bin to zero, as here: <a href="https://stackoverflow.com/q/29828825/2988730">Matplotlib cumulative histogram - vertical line placement bug or misinterpretation?</a>. Rather than simply ending the histogram, I'd like to be able to extend the line from the last bin to infinity at y=1.0.</p>