Скачать или смотреть Creating a Combination Chart in Google Data Studio that involves a calculated metric.

Create a Combination Chart in Google Data Studio that involves a calculated metric, blending data and metric level advanced filters.

In general, it encompasses these four areas:
Blending data amongst the same data source, using a JOIN key (a shared dimension) which will be the Month.
Creating two individual metrics that only contain the unique events of the event label you're interested in. This will be achieved by using metric level filters that filter by the event labels you're interested in (e.g. a user entering the flow / a user finishing the flow).
Selecting your dimension, metric #1 (sessions) and creating a new field that is a calculation to generate metric #2 
Styling the chart so it looks like a combination chart and involves two data series being shown on the left and right axis.