The version of FusionCharts supports JSON in 3.10.0, it looks like they didn't supply documentation on using it in that version (they do for 3.10.0).
You can confirm this by using the "Chart" control located under the "Advanced" menu within a section. Those charts load up data in a JSON format.
Is there a way to pass data in JSON format to Fusion chart control?
The control we ship for custom charts only supports XML currently.
The function exists in latest version of Fusion Chart v 3.12.2. Can we upgrade Fusion library to the latest in Pega 7.2.1?
Upgrading could have some complications and I would not recommend it. Fusion is usually pretty good about being backwards compatible but we've had to fix some issues we've seen in the past for the charts we auto-generate when we upgrade.