categoryAxis.GapWidth = 3;
categoryAxis.Labels.Add("A");
categoryAxis.Labels.Add("B");
categoryAxis.Labels.Add("C");
categoryAxis.Labels.Add("D");
model.Axes.Add(categoryAxis);
var linearAxisSoft = new LinearAxis();
linearAxisSoft.Key = "y1";
linearAxisSoft.AbsoluteMinimum = 0;
linearAxisSoft.Position = AxisPosition.Left;
model.Axes.Add(linearAxisSoft);
var linearAxisSoft1 = new LinearAxis();
linearAxisSoft1.Key = "y2";
linearAxisSoft1.AbsoluteMinimum = 0;
linearAxisSoft1.Position = AxisPosition.Right;
model.Axes.Add(linearAxisSoft1);
BarSeries barSeries = new BarSeries();
barSeries.XAxisKey = "y1";
barSeries.YAxisKey = "x";
barSeries.Background = OxyColors.Green;
barSeries.FillColor = OxyColors.YellowGreen;
barSeries.IsStacked = true;

barSeries.Items.Add(new BarItem (60,0));
barSeries.Items.Add(new BarItem(30, 1));
barSeries.Items.Add(new BarItem(60, 2));
barSeries.Items.Add(new BarItem(30, 3));
model.Series.Add(barSeries);
LineSeries lineSeries = new LineSeries();
lineSeries.MarkerType = MarkerType.Square;
lineSeries.MarkerFill = OxyColors.Red;
lineSeries.MarkerSize = 20;
lineSeries.XAxisKey = "x";
lineSeries.YAxisKey = "y2";
lineSeries.Points.Add(new DataPoint(0, 40));
lineSeries.Points.Add(new DataPoint(1, 30));
lineSeries.Points.Add(new DataPoint(2, 60));
lineSeries.Points.Add(new DataPoint(3, 30));
model.Series.Add(lineSeries);
return model;
代码如上：在Serires重叠的时候，Tracker的时候如何显示最上面的Series呢