When creating a dashboard, you can add multiple data by a) processing a new data using one of the data connectors or b) adding data from existing analysis results.
First Click on the "manage data" icon in the top options bar,
then select either "New Analysis" or "Existing Analysis Results".
Click on the data you want to add, and they will be listed on the right. Once you complete adding the data sets, click on "Create New Dashboard". All the data sets you selected will be visualized in the same dashboard.