D'oh! That's my mistake there. The simple way to handle the problem is to clear the selection in the main chart after someone clicks on it (see this:
http://jsfiddle.net/asgallant/x5Eth/4/). In doing so, you lose the visual indication of which column was selected in the main chart. If that doesn't matter to you, then you're good to go. If you want to keep the visual indication in the main chart, then you have to decide what should happen when someone clicks a column a second time (the second click clears the selection, which is why the script throws an error when you try to access information about the selected element). This simplest case here is to do nothing (see this:
http://jsfiddle.net/asgallant/x5Eth/5/).