The method getValueForDisplay() in the class CategoryAxis checks the wrong boundaries, i.e. for a horizontal axis, the max is the height and not the width. Same mix up for a vertical axis where the max is the width, not the height.

See following code for clarification:
{code}
    @Override public String getValueForDisplay(double displayPosition) {
        if (getSide().equals(Side.TOP) || getSide().equals(Side.BOTTOM)) { // HORIZONTAL
            if (displayPosition < 0 || displayPosition > getHeight()) return null; // <-------------- WRONG SHOULD BE displayPosition > getWidth()
            double d = (displayPosition - firstCategoryPos.get()) / categorySpacing.get();
            return toRealValue(d);
        } else { // VERTICAL
            if (displayPosition < 0 || displayPosition > getWidth()) return null; // <-------------- WRONG SHOULD BE displayPosition > getHeight()
            double d = (displayPosition - firstCategoryPos.get()) / (categorySpacing.get() * -1);
            return toRealValue(d);
        }
    }
{code}