I finally figured it out. One incomplete chart was causing the trouble. It was listed in the wp_posts table in the database, but not in the wp_postmeta table. I deleted it from the wp_posts table and everything returned to normal.
To add to jcduron’s last comment: For me, the tooltips with the numbers are functional when I first import the data, but they stop working when I save the chart, even if I never touch the Advanced button.
