Hi Mantri,
Calling two services is not good for performance if you data is too much ... means if you have not a lot records from the backend its fine , it will not impact much. I think most of the data will be in first service query that you can't stop from executing as it will require to show the graph when you load the app and second request on clicking is child data so I don't think so it will have much data.
But its better to use navigation property in OData - its like joining of two tables through foreign key .. in that you will not have to call the service on clicking the graph1 data points, you just need to attach it ... other things will automatically take care you just need to use routing as Sebastian show you in his comment.
thanks
Viplove