Add a property that would turn on the background color for custom symbols. See forum post: http://highslide.com/forum/viewtopic.php?f=9&t=7134
I agree this would be a useful feature addition.
Mike Eggleston commented
This would definitely be very helpful, especially with a bar/column graph that you would like to have an icon at the end of the bar/column (to represent an idea or group).