private List subplots;

    /** Total weight of all charts. */
    private int totalWeight;

    /** The gap between subplots. */
    private double gap;

    /** Temporary storage for the subplot areas. */
    private transient Rectangle2D[] subplotArea;  // TODO: move to plot state

    /**
     * Default constructor.
     */
    public CombinedRangeCategoryPlot() {
        this(new NumberAxis());