What is required for a single chart to reference data from two different data sources?

To reference data from two different data sources within a single chart in Data Studio, you must create a blended data source. Blending allows you to combine data from multiple sources based on a common join key (dimension). This creates a new, virtual data source that the chart can then use. Attaching both data sources to the report is necessary but not sufficient; it only makes them available. Selecting dimensions from the chart data tab only works if the chart is already connected to a single data source containing all necessary fields. Adding a data control allows users to filter data but doesn't combine sources for a single chart.