Excel 2007 : Please tell me why I get a "Run-time error '9' Subscript out of range"

Hello again!

In the code below I am attempting to add a new series to a chart, after the user defines the Y-axis value to use in drawing the line. They get three lines to define (High Alarm, Set Point and Low Alarm) and I have only added the new series code to the first possibility, where they entered a value in cell $D$4. Note: The graph is on the sheet "Cond Graph" as is the cells where they enter the y-axis values, but the data used by the graph is on the sheet "AIO-Cond".

When I enter a new value into "Cond Graph" cell $D$4, the code below halts with an error:
"Run-time error '9':
Subscript out of range"

And the line in the code below that is highlighted by the Debug is (not quite halfway down):
"With Worksheets("Cond Graph")"

Sub Worksheet_Change(ByVal Target As Range)
    
Application.ScreenUpdating = False
Application.Calculation = xlCalculationManual
    
    ' Find the Row number for the last cell in the imported data, put in "LastRow"
    With Worksheets("Data Import").Range("A1")
        LastRow = LastInRow(Worksheets("Data Import").Range("A1"))
    End With
    
    ' Now copy the user defined value to the appropriate column on the device data page
    ' and create new Series on chart for user defined limit line.
    If Target.Address = "$D$4" Then
        With Worksheets("AIO-Cond")
            .Range(.Range("L4"), .Range("L" & LastRow)).Value = Target.Value
        End With
        With Worksheets("Cond Graph")
            Worksheets("Cond Graph").ChartObjects("Chart 1").Activate
            ActiveChart.SeriesCollection.NewSeries
            ActiveChart.SeriesCollection(2).Name = "='Cond Graphs'!$C$4"
            ActiveChart.SeriesCollection(2).Values = _
            "='AIO-Cond'!$A$4:$A & LastRow,'AIO-Cond'!$B$4:$B &LastRow,'AIO-Cond'!$L4:$L & LastRow"
        End With
    End If

    If Target.Address = "$D$5" Then
        With Sheets("AIO-Cond")
            .Range(.Range("M4"), .Range("M" & LastRow)).Value = Target.Value
        End With
    End If

    If Target.Address = "$D$6" Then
        With Sheets("AIO-Cond")
            .Range(.Range("N4"), .Range("N" & LastRow)).Value = Target.Value
        End With
    End If

Application.ScreenUpdating = True
Application.Calculation = xlCalculationAutomatic
End Sub

As usual, I have no idea what that error means, or more correctly why I am getting that error. Can any of you nice folks help?

- Thomas