Use the coordinate systems SetScaleStartY and/or SetScaleStopY method after the scale is created, or any auto-scaling calls for the coordinate system. It must be called before the axes are created, since the axes by default auto-scale to the referenced coordinate system.
CartesianCoordinates pTransform1 = new CartesianCoordinates( 0.0, 0.0, 1.0, 500.0);
pTransform1.SetGraphBorderDiagonal(0.05, 0.175, 0.08, 0.35) ;
pTransform1.SetScaleStartY(-500);
pTransform1.SetScaleStopY(750);
LinearAxis baraxis = new LinearAxis(pTransform1, ChartObj.Y_AXIS);
chartVu.AddChartObject(baraxis);