Thank you, JDungan! I thought it must be something simple. I had been making sure everything was unselected, but didn't know about clicking on the dot next to title. It turns out my graph was indeed locked, though I don't recall ever locking it that way. But unchecking "Lock Selection" resolved the ...

I have a freeform table with time down the column and several metrics across the top. I made a line graph visualization from this table. At first it looked fine and the graph spanned the full time period (last full 30 days, in this case). But then suddenly the line graph was only showing one day of ...