You need to remember screen id of your page and check current screen id before adding new menu item:
add_action( 'admin_menu', 'my_plugin_menu' );
function my_plugin_menu() {
global $my_swell_plugin_page;
$my_swell_plugin_page = add_menu_page( 'My Swell Plugin', 'My Swell Plugin', 'edit_posts', 'blah', 'generate_main_page' );
}
function generate_main_page() {
//We are definitely in our plugins page now - let's modify the admin toolbar.
}
add_action( 'admin_bar_menu', 'add_toolbar_items', 100 );
function add_toolbar_items( $admin_bar ) {
global $my_swell_plugin_page;
if ( $my_swell_plugin_page == get_current_screen()->id ) {
$admin_bar->add_menu( array(
'id' => 'my-item',
'title' => 'My Item',
'href' => '#',
'meta' => array(
'title' => __( 'My Item' ),
),
) );
}
}