code: def diffp(self, diffcol: str, name: str="Diff"):
          
          try:
              df = self.df.copy()
              previous = 0
              i = 0
              vals = [df[diffcol].iloc[0]]
              for _, row in df.iterrows():
                  val = row[diffcol] - previous
                  new = round(val, 1)
                  previous = row[diffcol]
                  if i == 0:
                      vals = [0]
                  else:
                      vals.append(new)
                  i = 1
              self.df = df
              self.add(name, 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 previous value
  
          :param diffcol: column to diff from
          :type diffcol: str
          :param name: diff column name, defaults to "Diff"
          :type name: str, optional
  
          :example: ``ds.diffp("Col 1", "New col")``