Where does y2 field come from in stacked bar example

[Skorpeo]

I am looking at the stacked bar example : https://github.com/wrobstory/vincent/blob/master/examples/stacked_bar_examples.py

and I am having trouble understanding the following snippet:

enter_props = PropertySet(x=ValueRef(scale='x', field="data.idx"),
y=ValueRef(scale='y', field="y"),
width=ValueRef(scale='x', band=True, offset=-1),
y2=ValueRef(field='y2', scale='y'),
fill=ValueRef(scale='color', field='data.col'))

Specifically I don't understand:

 y2=ValueRef(field='y2', scale='y')

I don't understand it because I can't find a field called "y2" in the data set so how is this working??