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")``

def diffp(self, diffcol: str, name: str="Diff"):
        """
        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")``
        """
        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")