Using amCharts, how can I recreate thresholds behind bars as shown below?

I've been given the image above as an example of what the final chart should look like, but I'm having difficulty creating it.

We have to use amCharts due to the technology we're using, and I've got the columns stacked and coloured as I want, but I can't find a way to add the thresholds (shown in light green/blue/yellow). It'd also be nice to display it on a logarithmic scale as per the top of the image, but I'm not quite so bothered about that.

Any help appreciated.