Double-Y chart is a line chart that can show the relationship between data sources of two different units and two axis which can be used to show data at different scale for example, thousands versus millions on the same chart. You can then compare the correlation between the two data sources with the units taken out of the equation.
Select the data source that you wish to get insights on, and apply filters as required. Please note that when you choose specific measures, select only measures of same units. At this point, you can filter teams and focus areas only by plans, you can filter the other entities by Types, Health, and so on. Remember that, if you're choosing different entities, then you'll not be able to filter by Tags. While you can choose different combinations of data in the Data Arrangement section, here's a use case to give you more clarity.
If you want to evaluate if there is correlation between a KPI and an objective, this chart can help with that. By selecting Increase supply chain efficiency as one data source and Average progress of Measures (which represents the actual quantitative value of the KPI), and select Improve operational efficiency as the second data source and Average progress of Objectives (which represents the progress of the objective), you can then see if the the KPI is actually correlated or driving the progress of the objective. Toggle the Display Numbers if you wish to see the values on the chart.
Hover the mouse over the lines to see the details, and click inside the chart to open the chart context table - you'll see the details like owner, due date, progress, health of the objective/measure/action, and the relevant plan. You can sort the fields, the way you want. If the objectives are chosen, you can further expand or drilldown to view its associated success criteria. Click into any of these rows to open the relevant entity from the Sidebar.