If you want to insert a horizontal average line to a chart, you can calculate the average of the data first, and then create the chart. Please do as this:

1. Calculate the average of the data with Average function, for example, in Average Column C2, type this formula: =Average($B$2:$B$8), and then drag this cell's AutoFill handle to the range as you need. See screenshot:

Note: Click to know more about applying the same formula to entire column, or applying the exact same formula/value to entire column without cell number incrementing.

2. And then select this range and choose one chart format that you want to insert, such as 2-D Column under the Insert tab. See screenshot:

3. And a chart has been created, please click one of the average data column (the red bar) in the chart, right click and select the Change Series Chart Type from the context menu. See screenshot:

4. In the popped out Change Chart Type dialog, click to highlight the Combo in the left bar, click box behind the Average, and then select the a line chart style from the drop down list. See screenshot:

5. Click the OK button. Now, you have a horizontal line representing the average in your chart, see screenshot:

Demo: add a horizontal average line to a chart with a helper column in Excel

2 Clicks to add a horizontal average line to a column Chart

If you need to add a horizontal average line to a column chart in Excel, generally you need to add the average column to the source data, then add the data series of averages to the chart, and then change the chart type of the new added data series. However, with the Add Line to Chart feature of Kutools for Excel, you can quickly add such an average line in a chart by only 2 steps!

Supposing you have created a column chart with your data in your worksheet, and the following VBA code also can help you to insert an average line across your chart.

1．Click one of the data column in your chart, and then all the data columns will be selected, see screenshot:

2. Hold down the ALT + F11 keys, and it opens the Microsoft Visual Basic for Applications window.

3. Click Insert > Module, and paste the following code in the Module Window.

VBA: add an average line to the chart

Sub AverageLine()
'Update 20130907
Dim ser As Series
Dim arr As Variant
Dim total As Double
Dim outArr As Variant
If VBA.TypeName(Application.Selection) <> "Series" Then Exit Sub
Set ser = Application.Selection
arr = ser.Values
total = Application.WorksheetFunction.Average(arr)
ReDim outArr(LBound(arr) To UBound(arr))
For i = LBound(outArr) To UBound(outArr)
    outArr(i) = total
Next
With ActiveChart.SeriesCollection.NewSeries
    .XValues = ser.XValues
    .Values = outArr
    .Name = "Average " & ser.Name
    .AxisGroup = ser.AxisGroup
    .MarkerStyle = xlNone
    .Border.Color = ser.Border.Color
    .ChartType = xlLine
    .Format.Line.ForeColor.ObjectThemeColor = msoThemeColorAccent6
End With
End Sub

4. Then press F5 key to run this code, and a horizontal average line has been inserted into the column chart. See screenshot:

Note: This VBA only can run when the Column format you insert is 2-D Column.

This method will recommend an amazing tool, Add Line to Chart feature of Kutools for Excel, to quickly add a horizontal average line to the selected column chart with 2 clicks only!

Kutools for Excel- Includes more than 300 handy tools for Excel. Full feature free trial 30-day, no credit card required! Get It Now

Supposing you have created a column chart as below screenshot shown, and you can add a horizontal average line for it as follows:
 

1. Select the column chart, and click Kutools > Charts > Add Line to Chart to enable this feature.

2. In the Add line to chart dialog, please check the Average option, and click the Ok button.

Now the horizontal average line is added to the selected column chart at once.

Insert and print average on each page in Excel

Kutools for Excel's Paging Subtotals utility can help you to insert all kinds of subtotals (such as Sum, Max, Min, Product, etc.) in every printed page easily.