To use an existing data source in the current dashboard, follow these steps in the Dashboard Designer:
Click on the Data Source button in the configuration panel. This will open the data panel.

In the data panel window, click on the USE EXISTING option. This will open the EXISTING DATA SOURCE window.

Select the required data source that you want to include in the current dashboard. Once selected, click on OK.

After following these steps, the respective data source(s) will be included in the DATA SOURCES window as shown below.

NOTE: If you include a data source with the same name, a warning message will be displayed.
