To show the data in the chart, you basically need to store data in either persistable or non-persistable entity such that there are attributes in the entity which you set as data attributes for chart axes (x-axis data and y-axis data).
https://docs.mendix.com/refguide/charts-configuration Please see this chart configuration in docs to understand how you can set data source for a chart using any entity. Now when you REST API call will store data in the entity, chart is set to use that entity as the data source, your chart starts displaying the data.
You can set chart refresh interval to update it with new data.
To store JSON data received, you can make use of JSON import mappings.