Pasted from documentation.

#if set to true the grid initially is hidden. The data is not loaded (no request is sent) and only the caption layer is shown. When the show/hide button is clicked the first time to show grid, the request is sent to the server, the data is loaded, and grid is shown. From this point we have a regular grid. This option has effect only if the caption property is not empty and the hidegrid property (see below) is set to true.

$grid["hiddengrid"] = true;
$g->set_options($grid);