public void computeScrollSurfaceSize(Point out) {
        out.set((int) (maxViewport.width() * contentRectMinusAllMargins.width() / currentViewport.width()),
                (int) (maxViewport.height() * contentRectMinusAllMargins.height() / currentViewport.height()));
    }
Computes the current scrollable surface size, in pixels. For example, if the entire chart area is visible, this
is simply the current size of {@link #contentRectMinusAllMargins}. If the chart is zoomed in 200% in both
directions, the
returned size will be twice as large horizontally and vertically.
hellocharts-library/src/lecho/lib/hellocharts/computator/ChartComputator.java