Final answer:
In Lightning Experience, a saved joined report with a chart, created using the 'lightning experience report builder', is required to be used as the data source for a dashboard component.
Step-by-step explanation:
In Lightning Experience, to use a joined report as the data source for a dashboard component, you need to ensure that the joined report meets certain criteria. First, the report must be saved with the 'lightning experience report builder'. Also, the report must contain a chart. Without a chart, the report cannot be used as a source for a dashboard component, since dashboard components display data visually. Additionally, when you want to use a joined report in a dashboard, you must add the report chart to the dashboard, not the report itself. This means that the chart must be set up and properly configured in the joined report before you attempt to add it to the dashboard.