code: def _inc_lat(self, latitude, delta):
          
          y = self._convert_latitude(latitude)
          y += delta
          return 360 / pi * atan(
              exp((180 - y * 360 / (2 ** self._zoom) / self._size) * pi / 180)) - 90
docs: Shift the latitude by the required number of pixels (i.e. text lines).