I think the best approach for this scenario is to have a composite chart with a stacked column layer and a line layer. Here's an example:

private void Form1_Load(object sender, EventArgs e)
{
    //data
    DataTable dt = new DataTable();
    dt.Columns.Add("col1", typeof(string));
    dt.Columns.Add("col2", typeof(double));
    dt.Columns.Add("col3", typeof(double));
    dt.Rows.Add(new object[] { "item1", 5, 4 });
    dt.Rows.Add(new object[] { "item2", 2, 6 });
    dt.Rows.Add(new object[] { "item3", 4, 7 });

    ultraChart1.ChartType = ChartType.Composite;
    ChartArea area = new ChartArea();
    ultraChart1.CompositeChart.ChartAreas.Add(area);

    //create 2 stacked series
    NumericSeries series1 = new NumericSeries();
    series1.Data.DataSource = dt;
    series1.Data.LabelColumn = "col1";
    series1.Data.ValueColumn = "col2";
    series1.Label = "series1";
    series1.DataBind();
    ultraChart1.CompositeChart.Series.Add(series1);

    NumericSeries series2 = new NumericSeries();
    series2.Data.DataSource = dt;
    series2.Data.LabelColumn = "col1";
    series2.Data.ValueColumn = "col3";
    series2.Label = "series2";
    series2.DataBind();
    ultraChart1.CompositeChart.Series.Add(series2);

    //create the axes for stacked chart
    AxisItem xAxis = new AxisItem(ultraChart1, AxisNumber.X_Axis);
    xAxis.DataType = AxisDataType.String;
    xAxis.SetLabelAxisType = SetLabelAxisType.GroupBySeries;
    xAxis.Labels.ItemFormat = AxisItemLabelFormat.ItemLabel;
    area.Axes.Add(xAxis);

    AxisItem yAxis = new AxisItem(ultraChart1, AxisNumber.Y_Axis);
    yAxis.DataType = AxisDataType.Numeric;
    yAxis.Labels.ItemFormat = AxisItemLabelFormat.DataValue;
    area.Axes.Add(yAxis);

    //stacked layer
    ChartLayerAppearance stackLayer = new ChartLayerAppearance();
    stackLayer.ChartType = ChartType.StackColumnChart;
    stackLayer.AxisX = xAxis;
    stackLayer.AxisY = yAxis;
    stackLayer.Series.Add(series1);
    stackLayer.Series.Add(series2);
    stackLayer.ChartArea = area;
    ultraChart1.CompositeChart.ChartLayers.Add(stackLayer);

    //create an axis for the line chart as it's different from the column axis
    AxisItem xAxisLine = new AxisItem(ultraChart1, AxisNumber.X_Axis);
    xAxisLine.DataType = AxisDataType.String;
    xAxisLine.SetLabelAxisType = SetLabelAxisType.ContinuousData;
    area.Axes.Add(xAxisLine);

    //you can calculate the average here.
    double average = 12;

    //create a line series
    NumericSeries lineSeries = new NumericSeries();
    for (int i = 0; i < series1.Points.Count; i++)
    {
        lineSeries.Points.Add(new NumericDataPoint(average, "", false));
    }
    lineSeries.Label = "average";
    lineSeries.PEs.Add(new PaintElement(Color.Red));
    ultraChart1.CompositeChart.Series.Add(lineSeries);

    //create a line layer
    ChartLayerAppearance lineLayer = new ChartLayerAppearance();
    lineLayer.ChartType = ChartType.LineChart;
    lineLayer.AxisX = xAxisLine;
    lineLayer.AxisY = yAxis;
    lineLayer.Series.Add(lineSeries);
    lineLayer.ChartArea = area;
    ultraChart1.CompositeChart.ChartLayers.Add(lineLayer);
}