You can use a join sql query to join the 2 tables then use the Map process to split your "marks" column to multiple column like this:

$this->src(...)
->query($joinQuery)
->pipe(new \koolreport\processes\Map(array(
    "{value}" => function($row) {
        $marks = explode(",", $row["marks"]);
        foreach ($marks as $i => $mark) {
            $row["mark_" . $i] = $mark;
        }
        return $row;
    }
)))
->pipe($this->dataStore("marks"));

Then your datastore "marks" would have "mark_1", "mark_2", "mark_3", etc columns to draw chart together with column "student_id".