@Override
            public void onViewportChanged(Viewport newViewport) {
                // don't use animation, it is unnecessary when using preview chart.
                chart.setCurrentViewport(newViewport);
            }
Viewport listener for preview chart(lower one). in {@link #onViewportChanged(Viewport)} method change
viewport of upper chart.
hellocharts-samples/src/lecho/lib/hellocharts/samples/PreviewLineChartActivity.java