title: Altair - draw a line in plot where x = y
body: <p>This snippet:</p>
  <pre><code>chart = alt.Chart(df).mark_point(filled=True).encode(
        alt.X('Goals Conceded:Q'),
        alt.Y('Goals:Q'),
        alt.Size('Goals:Q', legend=None, scale=alt.Scale(range=[0, 1500])),
        alt.Color('Color', legend=None, scale=None),
        tooltip = [alt.Tooltip('For Team:N'),
                  alt.Tooltip('Goals:Q'),
                  alt.Tooltip('Goals Conceded:Q')]
        ).properties(
            width=800,
            height=600
        )
  </code></pre>
  <p>Plots:</p>
  <p><a href="https://i.stack.imgur.com/L7mNu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/L7mNu.png" alt="enter image description here" /></a></p>
  <p>Now I would like to manually add a line, where x = y, in order to get the following result:</p>
  <p><a href="https://i.stack.imgur.com/IFqZN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/IFqZN.png" alt="enter image description here" /></a></p>
  <hr />
  <p>How can I do this?</p>