There is no direct way, you can try below methods:
1. If you want to compare existing charts with new charts ( from regression point) , you can save earlier charts and can use sikuli to match it with the current chart. You can use sikuli in your selenium code by importing jars . But you will not be able to test it for current values .
2. If your application is using fusion charts , there is a methode called getdataascsv , or getdataasxml , these methods display the values of the charts and tool tips , you can ask your developer to include these methods and put an alert on it. In your selenium , you can use javascript executor and can call these functions and can pull the values from alert.
Thanks
Harshvardhan