At line 188 of Axis.js there is a hard coded check for the Bar series.
Code like this prevents users creating their own series. You can see this by just duplicating the Bar series, giving it a new name and trying to use it. This code should be checking properties (or if JS was 'ready'), an interface.
I need to create a overlaid bar series, something thats been available since the early days of Flex 3.