Hi,

Sorry for my late reply, you can add the role like this:

    Timeline::create(array(
        "dataSource"=>$this->dataStore("data"),
        "columns"=>array(
            "President",
            "Start"=>array(
                "type"=>"date",
            ),
            "End"=>array(
                "type"=>"date",
            ),
            "myTooltip"=>array(
                "type"=>"string",
                "role"=>"tooltip",
            )
        )
    ));