title: Is there a way to make a bubble plot in Altair which has circles that aren't filled?
body: <p>I am trying to develop a new data visualization/graphic, and the bubble plot available here is very similar to what I am trying to make in shape:</p>
  <p><a href="https://altair-viz.github.io/gallery/table_bubble_plot_github.html" rel="nofollow noreferrer">https://altair-viz.github.io/gallery/table_bubble_plot_github.html</a></p>
  <p>However, the graph I am trying to make involves some shaded bubbles and some filled in. Is there a way to edit this graph so that the bubble marks are not always filled?</p>
  <p>Thank you!</p>