title: Adding the line of identity to a scatter plot using altair
body: <p>I have created a basic scatter plot to compare two variables using altair. I expect the variables  to be strongly correlated and the points should end up on or close to the line of identity.</p>
  
  <p>How can I add the line of identity to the plot?</p>
  
  <p>I would like it to be a line similar to those created by <code>mark_rule</code>, but extending diagonally instead of vertically or horizontally.</p>
  
  <p>Here is as far as I have gotten:</p>
  
  <pre class="lang-py prettyprint-override"><code>import altair as alt
  import numpy as np
  import pandas as pd
  
  norm = np.random.multivariate_normal([0, 0], [[2, 1.8],[1.8, 2]], 100)
  
  df = pd.DataFrame(norm, columns=['var1', 'var2'])
  
  chart = alt.Chart(df, width=500, height=500).mark_circle(size=100).encode(
      alt.X('var1'),
      alt.Y('var2'),
  ).interactive()
  
  line = alt.Chart(
      pd.DataFrame({'var1': [-4, 4], 'var2': [-4, 4]})).mark_line().encode(
              alt.X('var1'),
              alt.Y('var2'),
  ).interactive()
  
  chart + line
  </code></pre>
  
  <p>The problems with this example is that the line doesn't extend forever when zooming (like a rule mark) and that the plot gets automatically scaled to the line endings instead of only the points.</p>