public Font getTickLabelFont(Comparable category) {
        if (category == null) {
            throw new IllegalArgumentException("Null 'category' argument.");
        }
        Font result = (Font) this.tickLabelFontMap.get(category);
        // if there is no specific font, use the general one...
        if (result == null) {
            result = getTickLabelFont();
        }
        return result;