code: def values(self):
          
          lower = float(self.lowerSpnbx.value())
          upper = float(self.upperSpnbx.value())
          return (lower, upper)
docs: Gets the user enter max and min values of where the 
          raster points should appear on the y-axis
  
          :returns: (float, float) -- (min, max) y-values to bound the raster plot by