"Could you try to deactivate all plugins, one at a time, and let me know if one of them conflicts with Jetpack?"
Okay, plugins I have:
Akismet
Bulletproof Security
Custom Permalinks
Jetpack by Wordpress.com
Marketpress
Membership Premium
Page-list
Page Links To
Pay Per View
Secure HTML 5 Video Player
WP-CopyProtect
WP Editor
WPMU DEV Dashboard
One at a time, I deactivated each one, then refreshed the site.
At no point in time did the sparkline reappear -- the link always appeared broken.