[flexcoders] Generic Axis class?

[netdeep]

I am creating a chart where the user can specify what type of horizontal axis 
he desires. 
Right now, I'm trying to allow both CategoryAxis and DateTimeAxis options. But 
in the API it 
seems like these two do not share a useable, generic ancestry. I tried AxisBase 
and IAxis, but 
neither worked. 

I'd like to hypothetically declare the following:

var hAxis:GenericAxis; // What class type should this be?

// then instantiate
hAxis = new DateTimeAxis();

// or
hAxis = new CategoryAxis();