Returns a reference to the Axis object that is the "base"
(independent axis) from which the values are drawn.
- pane (GraphPane)
This property is determined by the value of Base for
BarItem, ErrorBarItem, and HiLowBarItem
types. It is always the X axis for regular LineItem types.
Note that the Base setting can override the
IsY2Axis and YAxisIndex settings for bar types
(this is because all the bars that are clustered together must share the
same base axis).
