How do I update the contents of a placeholder with a generated image in Rails?

send_data g.to_blob, :disposition=>'inline', :type=>'image/png', :filename=>'top_n.pdf'

:complete => "updateImg(id_of_div, request.responseText)"

function updateImg(id, img)
{
  $(id).innerHTML = '<img src="' + img + '" />';
}

def graph
    # get the relevant data for 'g' here
    respond_to do |format|
        format.html #uses the default view if relevant (good for debugging)
        format.png do
            send_data g.to_blob,
                :disposition=>'inline',
                :type=>'image/png',
                :filename=>'top_n.pdf'
        end
    end
end

<img src="controller/graph.png" alt="Something"/>