code: def diffn(self, diffcol: str, name: str="Diff"):
          
          try:
              vals = []
              i = 0
              for _, row in self.df.iterrows():
                  current = row[diffcol]
                  try:
                      nextv = self.df[diffcol].iloc[i + 1]
                  except Exception:
                      vals.append(nan)
                      continue
                  val = nextv - current
                  vals.append(round(val, 1))
                  i += 1
              self.add("Diff", vals)
          except Exception as e:
              self.err(e, self._append, "Can not diff column")
              return
          self.ok("Diff column " + name + " added to the dataframe")
docs: Add a diff column to the main dataframe: calculate the diff
          from the next value
  
          :param diffcol: column to diff from
          :type diffcol: str
          :param name: diff column name, defaults to "Diff"
          :type name: str, optional
  
          :example: ``ds.diffn("Col 1", "New col")``