Codeigniter Highchart library series update

Code:
function _dataInst($id_inst)

    {

                 $users=$this->functions->show_users($id_inst);

                 foreach ($users->result() as $row)  

                 {

                     $point[]=rand(1,500);

                     $data['users']['data'] = rand(100, 500);   

                     $data[$users]['name'] = $row->surname;



                 }

                $data['users']['data'] =$point;

                $data['axis']['categories']=$users;

                $data['axis']['labels']=array('rotation'=>-45);

        return $data;

    }



private function getChartInst($id_inst) {

             $graph_data = $this->_dataInst($id_inst);    

                $this->highcharts->set_type('column');

                $this->highcharts->set_title('Title', 'Subtitle'); 

                $this->highcharts->set_xAxis($graph_data['axis']);  

                $this->highcharts->set_serie($graph_data['users']); 

                $this->highcharts->render_to("chart");

                return $data['chartsInst'] = $this->highcharts->render();

        }