public void setViewportChangeListener(ViewportChangeListener viewportChangeListener) {
        if (null == viewportChangeListener) {
            this.viewportChangeListener = new DummyVieportChangeListener();
        } else {
            this.viewportChangeListener = viewportChangeListener;
        }
    }
Returns viewport for visible part of chart, for most charts it is equal to current viewport.

@return
hellocharts-library/src/lecho/lib/hellocharts/computator/ChartComputator.java