The Google Chart Tools | Pie Charts page shows an example of a concentric pie chart:

According to that page:
pc
A concentric pie chart.
Supply two or more data series.
Can someone provide an example on how to generate a chart similar to that in Ruby on Rails?
I don’t think there is a ruby library that supports Google’s concentric pie charts, but with a small hack you can make http://googlecharts.rubyforge.org/ work. Something like
will return the URL http://chart.apis.google.com/chart?chd=s:Yr3,ff9&cht=pc&chs=300×200&chxr=0,20,45|1,25,50 which is similar to what you want.