def add_axis(name, axis_class)
      axis = axis_class.new
      set_cross_axis(axis)
      axes << [name, axis]
    end
def add_axis(name, axis_class)
      axis = axis_class.new
      set_cross_axis(axis)
      axes << [name, axis]
    end
Adds an axis to the collection
 @param [Symbol] name The name of the axis
 @param [Axis] axis_class The axis class to generate