Please see the attached image (the image may be not fully shown in the post, please click the image to show it in a new window, there should be 3 charts in the image)
Hi Zach,
The chart actually doesn't currently draw a plot area border. What you are seeing in the one instance is that the chart chose to put a gridline at the top. You can get a better delineation between the plot area and the non plot area if you set the plotAreaBackground to a different color than the surrounding page. But I'm guessing a plot area border may be more desirable for you? I think we actually intended for a border color to be selectable by the user so I'm going to submit a bug to this effect.
-Graham