Sets the zoom scale

        :param pxms: number of pixels per ms
        :type pxms: int
        :returns: float -- the miliseconds between grid lines

def setPixelScale(self, pxms):
        """Sets the zoom scale

        :param pxms: number of pixels per ms
        :type pxms: int
        :returns: float -- the miliseconds between grid lines
        """
        pxms = float(pxms)/2
        self.pixelsPerms = pxms
        if pxms*self.gridms < GRID_PIXEL_MIN:
            self.gridms = self.gridms*2
        elif pxms*self.gridms > GRID_PIXEL_MAX:
            self.gridms = self.gridms/2
        self._viewIsDirty = True
        self.viewport().update()

        return self.gridms