code: def _minimal_y(self, p):
          
          y0 = self.pattern_y
          y1 = y0 + self._pixelsize(p)/2.
          return y0 if self._count_pixels_on_line(y0, p) < self._count_pixels_on_line(y1, p) else y1
docs: For the specified y and one offset by half a pixel, return the
          one that results in the fewest pixels turned on, so that when
          the thickness has been enforced to be at least one pixel, no
          extra pixels are needlessly included (which would cause
          double-width lines).