...apparently, font support for BLACK LARGE CIRCLE (U+2B24) is very limited. So you could include a font in your page that renders the symbol, or use another char.
I have a weird display problem with the legend:
Some of the chars will not display; even when i test them on a completely different page: http://jsfiddle.net/qro1ywyz/ . The first symbol will render correctly, the second doesn't.