'the transData of the axes to the world coordinate.
locator1, locator2 : grid locator for 1st and 2nd axis.
Derived must define "transform_xy, inv_transform_xy"
(may use update_transform)'

def __init__(self, extreme_finder, grid_locator1, grid_locator2, tick_formatter1=None, tick_formatter2=None):

    super(GridFinderBase, self).__init__()
    self.extreme_finder = extreme_finder
    self.grid_locator1 = grid_locator1
    self.grid_locator2 = grid_locator2
    self.tick_formatter1 = tick_formatter1
    self.tick_formatter2 = tick_formatter2
