Hi Jay,
As currently, there isn't a widget for charts, It's pretty certain to be added in future releases.
The Experience Builder is planned to catch up with the available widgets that exist for the WebApp Builder, but it's going to take a while.
Some other alternatives would be to either develop a widget or use an external resource, as you already did. I saw some Experiences that use external pie charts from different websites.
Shay.
Shay