You can use our additional solution Dashboard Designer to customize charts. You can find more information in the documentation.
To install Dashboard Designer for classic pages you should install it from SharePoint store. Please make sure that you have Scripting Capabilities turned on.
Click ‘Add an app’ link in the upper-right ribbon menu. Then go to SharePoint store in the menu to the left.
Search ‘Dashboard Designer’ there and install it on your site:
Then you should configure credentials for it.
Go to your site ‘Site contents’ and click on Dashboard Designer application.
Then enter your credentials in these fields:
Please keep in mind that you will need Site Owner rights.
In order to customize charts with Dashboard Designer navigate to the page containing charts and enter edit mode.
Clicking the link “Configure” will load Dashboard designer where you can customize your chart.