title: How can I position text relative to axes in altair?
body: <p>I'm trying to make a graph much like the <a href="https://altair-viz.github.io/gallery/multiline_tooltip.html" rel="nofollow noreferrer">Multi-Line Tooltip Example</a>. However I want to make two modifications. 
  First of all, I want to be able to zoom in to a portion of the data. I fixed that using <code>alt.selection(type="interval", encodings=["x"], bind="scales")</code> and a transform_filter. So far no problem.
  The problem is that the text labels near the points are overlapping because the lines are close together. Therefore I would like to move the labels to a fixed position within the axes along the top. Is it possible to put the labels at a fixed position within the axes, even when zooming in on the graph (see mockups below)? The problem is that when you zoom in both the x and the y domains change, so the positions of the labels should be expressed as a fraction of the domains.</p>
  
  <p>Another solution I could accept is when the selected value is appended to the legend labels, or some other label outside the plot area.</p>
  
  <p>Mock up of the full view:
  <a href="https://i.stack.imgur.com/d8xi2.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/d8xi2.png" alt="Full view"></a></p>
  
  <p>Mock up of the zoomed view:
  <a href="https://i.stack.imgur.com/kubfo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kubfo.png" alt="Zoomed view"></a></p>