Return the next available `c:ser/c:idx` value within the scope of
        this chart, the maximum idx value found on existing series,
        incremented by one.

def next_idx(self):
        """
        Return the next available `c:ser/c:idx` value within the scope of
        this chart, the maximum idx value found on existing series,
        incremented by one.
        """
        idx_vals = [s.idx.val for s in self.sers]
        if not idx_vals:
            return 0
        return max(idx_vals)+1