"I do not see any errors on your site at the moment."
Well, I've already checked for errors, so I would be surprised if you had. ;)
"...could you switch to a different theme for a few minutes, and see if that helps?"
This was the easiest thing to try at the moment, so I did that first.
1. Installed "twenty-twelve" theme (I only have one theme installed to reduce my vulnerability profile)
2. Activated "twenty-twelve" theme.
3. Reviewed site.
4. Sparkline appears!
5. Switched back to "Sliding Door" theme.
6. Sparkline disappears.
So, seems like it may be associated with the theme...?