title: Format text of mark_text in Altair
body: <p>I'm trying to create a chart somewhat along the lines of the <a href="https://altair-viz.github.io/gallery/multiline_tooltip.html" rel="noreferrer">Multi-Line Tooltip example</a>, but I'd like to format the string that is being printed to have some text added at the end. I'm trying to modify this part:</p>
  
  <pre><code># Draw text labels near the points, and highlight based on selection
  text = line.mark_text(align='left', dx=5, dy=-5).encode(
      text=alt.condition(nearest, 'y:Q', alt.value(' '))
  )
  </code></pre>
  
  <p>Specifically, rather than 'y:Q' I want something along the lines of 'y:Q' + " suffix". I've tried doing something like this:</p>
  
  <pre><code># Draw text labels near the points, and highlight based on selection
  text = line.mark_text(align='left', dx=5, dy=-5).encode(
      text=alt.condition(nearest, 'y:Q', alt.value(' '), format=".2f inches")
  )
  </code></pre>
  
  <p>Alternatively, I've tried:</p>
  
  <pre><code># Draw text labels near the points, and highlight based on selection
  y_fld = 'y'
  text = line.mark_text(align='left', dx=5, dy=-5).encode(
      text=alt.condition(nearest, f"{y_fld:.2f} inches", alt.value(' '))
  )
  </code></pre>
  
  <p>I think I see why those don't work, but I can't figure out how to intercept the value of y and pass it through a format string. Thanks!</p>