Thank you for the invaluable tool.
I have been running some tests on wpt.org since last 2-3 weeks. And on all tests, I do not see any graphs when on Test result data plots (graph_page_data.php).
I have checked this on Firefox 79, Chrome 85, Firefox Developement, Chrome Canary, as well as on latest Chrome/Brave on Android. Neither of these browsers shows any console errors.