private void setCurrentViewport(ChartComputator computator, float left, float top, float right, float bottom) {
        Viewport currentViewport = computator.getCurrentViewport();
        if (ZoomType.HORIZONTAL_AND_VERTICAL == zoomType) {
            computator.setCurrentViewport(left, top, right, bottom);
        } else if (ZoomType.HORIZONTAL == zoomType) {
            computator.setCurrentViewport(left, currentViewport.top, right, currentViewport.bottom);
        } else if (ZoomType.VERTICAL == zoomType) {
            computator.setCurrentViewport(currentViewport.left, top, currentViewport.right, bottom);
        }
    }
Smaller viewport means bigger zoom so for zoomIn scale should have value <1, for zoomOout >1
hellocharts-library/src/lecho/lib/hellocharts/gesture/ChartZoomer.java