Pls use the quarter function in your sql query and then the Cube process in your report:

//MyReport.php
$this->src("mysql")
->query("select quarter(activity_date) as 'activity_quarter', activity, ...") // other databases might have different quarter function from MySQL.
->pipe(new koolreport\cube\processes\Cube(array(
    "row" => "activity", 
    "column" => "activity_quarter",
    "count" => "actitivy",
)))
...

Let us know if we understand your question correctly. Tks,

Pls concat year(activity_date) and quarter(activity_date) to create Year Quarter column and group/cube by that. Rgds,