if (this.allowDuplicateXValues) {
                    // need to make sure we are adding *after* any duplicates
                    int size = this.data.size();
                    while (index < size 
                           && item.compareTo(this.data.get(index)) == 0) {
                        index++;
                    }
                    if (index < this.data.size()) {
                        this.data.add(index, item);
                    }
                    else {
                        this.data.add(item);
                    }
                }
                else {
                    throw new SeriesException("X-value already exists.");