This should do what you want:
add_action('generate_rewrite_rules', 'wpse128082_add_rewrite_rule');
function wpse128082_add_rewrite_rule($wp_rewrite) {
$new_rules = array(
'^charts/(.*)' => 'index.php?pagename=charts&something='.$wp_rewrite->preg_index(1),
);
$wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
}
add_filter('query_vars', 'wpse128082_add_query_var');
function wpse128082_add_query_var($query_vars) {
$query_vars[] = 'something';
return $query_vars;
}
On your Charts page, you can access the new query var with get_query_var('something')
.