def diffm(self, diffcol: str, name: str="Diff", default=nan):
        """
        Add a diff column to the main dataframe: calculate the
        diff from the column mean

        :param diffcol: column to diff from
        :type diffcol: str
        :param name: diff column name, defaults to "Diff"
        :param name: str, optional
        :param default: column default value, defaults to nan
        :param default: optional

        :example: ``ds.diffm("Col 1", "New col")``
        """
        try:
            df = self.df.copy()
            mean = self.df[diffcol].mean()
            vals = []
            for _, row in self.df.iterrows():
                num = row[diffcol]
                if num > 0:
                    diff = int(((num - mean) * 100) / mean)
                    vals.append(diff)
                else:
                    vals.append(default)
            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")