Add Joined report to the dashboard component as a source report
We can add Joined reports also to the dashboard as a source report. This feature is available with the winter’19 Salesforce release.
To use the joined report as a dashboard source report, joined report must have a chart.
To know more about this feature go to the URL: https://releasenotes.docs.salesforce.com/en-us/winter19/release-notes/rn_rd_dashboards_source_joined_reports.html