The legendmark option doesn't seem to be functional at the moment. Take a look at the code:
mark = paper[mark && mark.toLowerCase()] || "circle";
chart.labels = paper.set();
...
chart.labels[i].push(paper[mark](x + 5, h, 5).attr({ fill: clr, stroke: "none" }));
The legendmark is drawn using the Raphael paper function of the same name as the legendmark (e.g. "circle"). But if you specify a legendmark gRaphael tries to fetch the function twice - the "paper[mark]" parts above, resulting in an error as the code tries to call
undefined
as a function. This is the case even if you pass "circle" as the option value.There's a related pull request on github, but gRaphael development seems dormant.