That is what the GroupDataset.SetAutoScaleNumberGroups method is for. It tells the coordinate auto-scale routines to use only the specified number of groups in the calculation. In the example code below, the number of groups is limited to 1.
GroupDataset Dataset1 = new GroupDataset("Actual Sales",x1,y1);
Dataset1.SetAutoScaleNumberGroups(1);
CartesianCoordinates pTransform1 = new CartesianCoordinates();
pTransform1.AutoScale(Dataset1);