title: How to remove white border line of rectangles in Altair heatmap/mark_rect plots?
body: <p>I am trying to plot a heatmap with 400 rows and 12 columns, and the formatting of the borders of the rectangles makes the outcome look unexspected, see image below.</p>
  <pre><code>alt.Chart(df).mark_rect().encode(
    x='time_point:O',
    y=alt.Y('row_id',
            sort=alt.EncodingSortField(field=col,
            op=&quot;sum&quot;,
            order='ascending'),
            axis=None),
    color='binary_measure:N'
  ).properties(
    width=200,
    height=200
  )
  </code></pre>
  <p><a href="https://i.stack.imgur.com/FuaoR.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FuaoR.png" alt="Heatmap with unexpected border effects" /></a></p>
  <p>I realize it looks here like the borders are darker, but it's actually a white or lighter color line, see this reproducible example:</p>
  <pre><code>import altair as alt
  from vega_datasets import data
  
  source = data.cars()
  
  alt.Chart(source).mark_rect().encode(
        x='Cylinders:Q',
        y=alt.Y('Origin:N'),
        color='Horsepower:Q'
    ).properties(
        width=200,
        height=200
    ).configure_mark(
      strokeOpacity=0,
      strokeWidth=0,
      filled=True
    )
  </code></pre>
  <p>The result is this, with the lighter color borders highlighted.</p>
  <p><a href="https://i.stack.imgur.com/sMgWO.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/sMgWO.png" alt="Iris heatmap example with white/lighter borders highlighted." /></a></p>
  <p>In the code you can also see my attempts to remove that border, using <code>strokeOpacity=0</code>, <code>strokeWidth=0</code>, and <code>filled=True</code>, but to no avail.</p>
  <p>Any ideas how I can make the borders of the rectangles the same color as the fill color?</p>