Chart library that creates negative bar charts & ability to overlay second chart
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I know there are quite a few js & php chart libraries although I am not sure of the best method to create charts to look very similar to the image displayed below:

I am trying to create a chart that will allow me to use negative values - the tricky bit is the 'dark grey' overlay as I am not sure of the best library for doing this, I've used Google Charts but not had quite much success.

Any suggestions would be welcomed - thanks :)

![chart with negative values & secondary overlay values][1]
[1]: http://i.stack.imgur.com/H5cXU.jpg