To recap, this is how Wikipedia describes Sparklines:
Sparklines are a format developed by Edward Tufte for small graphic charts which can be inserted within text on a page.
Tufte describes sparklines as "data-intense, design-simple, word-sized graphics". Whereas the typical chart is designed to show as much data as possible, and is set off from the flow of text, sparklines are intended to be succinct, memorable, and located precisely where appropriate.
Theres an interesting proposal at the Mozilla Wiki which will combine sparklines with microsummaries. Microsummaries or "Live Titles" allow you to show a succint "summary" of the bookmarked page in your bookmarks menu and toolbar.
Together, the two go really well. This is what graphical microsummaries could look like:
Although it’s just a mockup, the screenshot looks absolutely beautiful and graphical microsummaries really could be a lot more useful than text-based ones.
The wiki page goes through some of the background to the idea and has some images of sparklines which give an indication of what graphic formats designers may choose to use.
This feature gets a +1 from me.
Addendum: Unicode already has special "block characters".
e.g. ▀▁▂▃▄▅▆▇█▉▊▋▌▍▎▏▐ ░ ▒ ▓ ▔▕▖▗▘▙ ▚ ▛ ▜▝ ▞ ▟