Another important thing to consider - did you build the charts the same way as you would build a widget in Piwik PRO dashboards? For the chart showing “returning visitors”, did you set it up by using the metric “Visitors” and making a scorecard filter like on the screenshots below?
Is my guess correct that you are comparing stats for a particular location? In the screenshot from Piwik PRO Analytics, I see the following text: “Country › Subdivision › City”, which suggests that you are looking at the Locations report.
If that’s the case, it’s important to make sure that you filter the data the same way in Google Data Studio. By default, Data Studio would return the average session time for all visitors without taking locations into account. If you add locations to the mix (like in Analytics reports), the data may be calculated differently (to split between cities and also to see results where location couldn’t be attributed).
In general, it can be pretty hard to compare a scorecard chart in GDS to a full report where the query for the data is more complex. A useful tip might be checking if the data is accurate by comparing it to a widget in Piwik PRO dashboards.
It is the same in my report. I do not even try to calculate the average session time in a table. The value itself is wrong. When i try to use the metric “session time” it somehow shows a huge number. So therefore i considered its the aggregated session time. There is no metric which is called “average session time”. I decided to calculate it by myself: “session time/sessions”. But the average is not the same as in Piwik Analytics.