Meanwhile, it's now even stranger. I switched to a different SD card yesterday and let the S9 format it, and this morning I saw the regular sleep report info on the machine's screen (for last night plus 1 week ago, 1 month, etc.), but when I looked at the card via ResScan, Feb. 25 (last night) was not there. The most recent date was the 24th, the previous night. Same result if I looked at statistics or summary graphs--last night was not showing up at all.
My PC showed me that about 3MB was used last night on the replacement SD card.
When I tried the new SD card in SleepyHead, last night's data was all there, and the graph scaling and traces were back to looking as they should look.