title: Add label to horizontal line annotation in Superset chart
body: <p>I see how to add a horizontal line to an Apache Superset chart, but can't figure out how to display the label on the chart.  I want users to be able to see it without having to mouseover the chart.</p>
  <p>Here's an example with the Games per Genre over time demo chart.  My annotation:</p>
  <p><a href="https://i.stack.imgur.com/oH63U.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/oH63U.png" alt="enter image description here" /></a></p>
  <p>That gets the line to display at 100, but its label - Target Number of Games - is not visible:
  <a href="https://i.stack.imgur.com/ArcDo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ArcDo.png" alt="enter image description here" /></a></p>
  <p>Is there a way to get the label to display just above or below its line?</p>