public TimePeriodAnchor getXPosition() {
        return this.xPosition;
    }

    /**
     * Sets the position within each time period that is used for the X values,
     * then sends a {@link DatasetChangeEvent} to all registered listeners.
     *
     * @param anchor  the anchor position (<code>null</code> not permitted).
     *
     * @see #getXPosition()
     */
    public void setXPosition(TimePeriodAnchor anchor) {
        if (anchor == null) {
            throw new IllegalArgumentException("Null 'anchor' argument.");
        }
        this.xPosition = anchor;
        notifyListeners(new DatasetChangeEvent(this, this));