How can I show transaction details bar chart?

How can i show total transaction group by month in chart js? I have a code where i can show total no of transactions in month. But I need total sum of transactions. Below is the code:

 $pie_data = order::select('total_amount' ,'created_at')->get()->groupBy(function($pie_data){
            return Carbon::parse($pie_data->created_at)->format('M');
        });

        $piemonths = []; 
        $piemonthTrans= []; 
        foreach($pie_data as $piemonth => $values){
            $piemonths[] = $piemonth;
            $piemonthTrans[] = count($values);
            
        }

When i change count function with sum , it is showing error - "Call to undefined function App\Http\Controllers\Admin\sum()"