American Heritage Dictionary
(1)
Century Dictionary
GNU Webster's 1913
(1)
WordNet
(1)
Elsewhere on the web
Last night Glenn Beck made crude drawings on a chalkboard, and tonight he and Bill O'Reilly used Barbie dolls to explain ACORN.— Megite Technology News: What's Happening Right Now
Chalk Media created the Mobile chalkboard, an application suite that enables the creation and deployment of Pushcasts, which carry video and audio files to BlackBerry devices. expected revenue for the fourth quarter of 2008 to be approximately 25\% lower than its third-quarter 2008 revenue of US$1. 585 billion.— SD Times Latest News
(The chalkboard is attached to the inside of your skull, its horrid cacaphony amplified with loudspeakers and by echoes.)— LinuxChix aggregator

Bubble size: how much this word was used in a year
Bubble height: used more or less than expected, vs. all uses evenly distributed
You can expect to see this word several times a year.