I solved this issue so am not posting in the Help section, but there seems to be a problem with PiHole's charts in the dashboard.
I changed the extension cord to my routers/PiHoles this morning and when the power came back up everything seemed to be working and Pi-Hole (v5.1.1) was reporting Total Query numbers but the charts were blank, showing a one-hour time range but no bars. I thought maybe it look an hour to update or so but many hours later it still was showing blank charts but ever-increasing client query counts in the boxes at the top of the dashboard.
After some googling I found recommendations to clear the browser cache and/or Ctrl+F5 on the page. Neither of these made any difference. I was using Chrome so I fired up Firefox instead and again the charts were blank on the logged-in dashboard, but the KPI boxes at the top were still counting up. So this seems to be a server-side issue and not a client browser/cache problem as it occurred on two different browsers.
I found another post by someone reporting that after a cold boot the charts hadn't appeared. After running a 'sudo shutdown -r now' on the PiHole Raspberry Pi (B 1) it came back up again and the charts were working in both browsers again.
I thought the developers might like to know and that this might be useful for others hitting the same issue. Note that it's possible that when I first turned the power back on this morning the PiHoles cam up before the Internet connection/routers they were connected to, which might be a reason PiHole got stuck in a funny mode with no charts, perhaps.