Compute colorbar offset and update separator line
        if map is non-zero.

def _update_separator(self, offset):
        """
        Compute colorbar offset and update separator line
        if map is non-zero.
        """
        offset_line = self.handles['offset_line']
        if offset == 0:
            offset_line.set_visible(False)
        else:
            offset_line.set_visible(True)
            if self.invert_axes:
                offset_line.set_xdata(offset)
            else:
                offset_line.set_ydata(offset)