Add chart example

examples/device-dashboard/main.c

@@ -7,6 +7,7 @@ void device_dashboard_fn(struct mg_connection *, int, void *, void *);
 
 int main(void) {
   struct mg_mgr mgr;
+  mg_log_set("3");
   mg_mgr_init(&mgr);
   mg_http_listen(&mgr, "http://0.0.0.0:8000", device_dashboard_fn, &mgr);
   for (;;) mg_mgr_poll(&mgr, 1000);