Create chart for user count of user role using chart module(c3 lib) in Drupal 8?
This is an issue of needing to be sure that you first set the format of the attachment display to "This chart attachment (override)", and then adjusting the format settings. Don't forget to attach your "Chart attachment" display to the appropriate display.

I wanted to set the style plugin to do this by default (see this issue: https://www.drupal.org/project/charts/issues/2899035), but I am unable to find any documentation on how to do this programmatically.