I really think that you are out of luck here. The trouble is that Charts are rendered server-side into static image files in PNG format.
If the image had been a nice format such as an inline canvas element, you could likely override the styles, removing the drop shadow, but this does not appear to be the case.
Hi Community! We're thrilled to share that Team Calendars for Confluence is now a built-in feature for Confluence Data Center releases 7.11 and beyond. A long time favorite, Team Cale...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events