code: def slope_percentile(self, date, mag):
          date_diff = date[1:] - date[:len(date) - 1]
          mag_diff = mag[1:] - mag[:len(mag) - 1]
          index = np.where(mag_diff != 0.)
          date_diff = date_diff[index]
          mag_diff = mag_diff[index]
          slope = date_diff / mag_diff
          percentile_10 = np.percentile(slope, 10.)
          percentile_90 = np.percentile(slope, 90.)
          return percentile_10, percentile_90