public double getRadius() {
        return this.radius;
    }
    
    /**
     * Sets the radius of the cap, as a percentage of the dial's framing
     * rectangle, and sends a {@link DialLayerChangeEvent} to all registered
     * listeners.
     *
     * @param radius  the radius (must be greater than zero).
     *
     * @see #getRadius()
     */
    public void setRadius(double radius) {
        if (radius <= 0.0) {
            throw new IllegalArgumentException("Requires radius > 0.0.");
        }
        this.radius = radius;
        notifyListeners(new DialLayerChangeEvent(this));