Every now and then I get emails/inquiries about using my simple "quick & dirty" charts. I always give permission, and in fact I never complain when other (usually more popular!) weblogs use them. Even on the very rare occasions where attribution is not given (I suspect this is usually an oversight in haste). This is fair enough, as I regularly use figures and tables from scientific papers in my blog posts. I believe this is all reasonably under "fair use."
But I've decided to explicitly assert that the charts I produce are under the Creatives Commons license.
This is not to force people to attribute. Even if someone uses my chart and does not attribute I won't really sue or anything like that (unless they're somehow miraculously making millions off them). Rather, it's a nudge to those who would use the charts, and should also make it so that people don't have to contact me directly via email or Twitter. If you want to use the chart or data, permissions is given implicitly.