The Google Analytics API only allows querying directly for measures and dimensions and does not allow Chartio to perform aggregations on the data.
Many Google Analytics metrics already aggregated. For example, Sessions is a count of sessions and Visitors is a count of visitors.
If you would like to apply aggregations on top of Google Analytics metrics, such as average of Sessions, you can do so in the Data Pipeline.
For definitions of individuals metrics, see Google’s Dimensions & Metrics Explorer.