Vaadin Timeline and Internet Explorer Line Graph Issue

I’m running into an issue with the line graph rendering in Internet Explorer 6-8. When trying to graph multiple datasets on the same timeline with lines, I’m noticing that only the first dataset is graphed as expected. All of the subsequent line graphs have this semi-transparent fill applied and I haven’t been able to figure out how to get rid of it. For example, here’s how the demo graph is displayed in Chrome/Firefox:

and here’s how it renders in IE (haven’t tried 9)

Is there a way to get rid of this fill or is this a bug?