public void setTickLabelPaint(Comparable category, Paint paint) {
        if (category == null) {
            throw new IllegalArgumentException("Null 'category' argument.");
        }
        if (paint == null) {
            this.tickLabelPaintMap.remove(category);
        }
        else {
            this.tickLabelPaintMap.put(category, paint);
        }
        notifyListeners(new AxisChangeEvent(this));