How to simply get the bounce rate through the API?



I am trying to learn a bit more about how to use the Piwik API in order to write some tutorials about it.
In one of them I would like to show how we can export the bounce rate for a given period into a Google spreadsheet. I just want the bounce rate, nothing more.
Here is my syntax:


so far, it works fine, but I would like to make it even more simple by not using the VisitsSummary method.
How can I do it? How can I just pick up a simple metrics?

Thank you very much for your answers.