What's a good icon to represent a legend or a key for a diagram?

I'm implementing a small web app widget that shows a legend/key for a diagram (or chart/map/graph/whatever), but only during mouseover of the widget. It's basically just going to be a div with a background image. What wo开发者_如何学编程uld be a good icon that communicates "I show the legend" or "I am the legend" to the unfamiliar users? I'm looking for something in the 10x10 to 20x20 pixel range.

Assuming that a legend is the same thing as a key (is it?) then I could use a small icon of a key (like you'd unlock a door with) but that seems cheesy or unclear at worst. I'm not sure that's really the clearest way to do it. Is there an icon that should instantly remind users of a legend?

In my region (southern USA) I would say the key is better known then legend. Thus I would find an icon of a key for this. There are many Icon sites one that I frequent is http://www.iconfinder.net/

Maybe just a question mark icon - help / legend are kinda similar on a map.

I'd use an (i) icon for "info". Or maybe a [+] icon, that's often used to represent "more"

Not that I'd use these directly, but maybe it'll spur some ideas.

Info http://salamottawafm.files.wordpress.com/2009/02/info_icon.png?w=48&h=48

Compass http://kuaizhixin.cn/Writable/ClientImages/PageMenuLegendIconLw.bmp

RedDot http://www.cityofoaksmarathon.com/assets/images/icon_red-dot.png

Something like a tourist information sign would probably be quite explicit to get information about an item...

I agree with chris_l that the i icon would probably be what you want. I am making this my own answer because I disagree so strongly with his second suggestion (to use a + icon). The + icon is already understood to either add something or expand something, so I think the user would be confused. For legends in my own apps, I use a blue circle with an italic, lower-case "i" in white. Everyone seems to get it.

At 10x10 or 20x20 pixels I think you're going to have to have something instantly recognisable. Don't over think this, if the obvious answer is a key, use a key