private function export($slug, Request $request, $format)
 {
     $chart = Chart::where('slug', $slug)->orWhere('id', $slug)->first();
     if (!$chart) {
         return App::abort(404, "No such chart");
     }
     $size = $request->input("size");
     if (!$size) {
         $size = "1000x700";
     }
     $split = explode("x", $size);
     $width = min(intval($split[0]), 3000);
     $height = min(intval($split[1]), 3000);
     $file = Chart::export($slug, $_SERVER['QUERY_STRING'], $width, $height, $format);
     return response()->file($file);
 }