
Thanks!
1 answer
For that you'd need to use a tool like Roll-Up Helper or Declarative Roll-Up by Any Fawcett (it's free!)
https://andyinthecloud.com/2016/06/19/declarative-rollup-tool-summer-release/
If you don't need to have the value stored in a Field on the Account you could create a Report with a Chart and use Embedded Analytics to display the information
Embedded Report Charts
https://help.salesforce.com/apex/HTViewHelpDoc?id=reports_embed_showcharts_pages.htm&language=en_US