Merril asked if there was a way to create a line chart so that when a line represented a negative value, the color of the line would change at the point when it went negative. For instance, in a particular data series, as long as the line represented positive values, it would be blue, but when the line represented negative values, it would change to red.

Unfortunately there is no way to easily do this in Excel. There are, however, a couple of workarounds you can try. The first is to use a macro to change the line colors of chart lines that represent negative values. The following macro is an example of such an approach:

Sub PosNegLine()
    Dim chtSeries As Series
    Dim SeriesNum As Integer
    Dim SeriesColor As Integer
    Dim MyChart As Chart
    Dim R As Range
    Dim i As Integer
    Dim LineColor As Integer
    Dim PosColor As Integer
    Dim NegColor As Integer
    Dim LastPtColor As Integer
    Dim CurrPtColor As Integer

    PosColor = 4 'Green
    NegColor = 3 'red
    SeriesNum = 1

    Set MyChart = ActiveSheet.ChartObjects(1).Chart
    Set chtSeries = MyChart.SeriesCollection(SeriesNum)
    Set R = GetChartRange(MyChart, 1, "Values")

    For i = 2 To R.Cells.Count
        LastPtColor = IIf(R.Cells(i - 1).Value < 0, NegColor, PosColor)
        CurrPtColor = IIf(R.Cells(i).Value < 0, NegColor, PosColor)

        If LastPtColor = CurrPtColor Then
            LineColor = LastPtColor
        Else
            If Abs(R.Cells(i - 1).Value) > Abs(R.Cells(i).Value) Then
                LineColor = LastPtColor
            Else
                LineColor = CurrPtColor
            End If
        End If
        chtSeries.Points(i).Border.ColorIndex = LineColor
    Next i
End Sub

Function GetChartRange(Ch As Chart, Ser As Integer, _
  ValXorY As String) As Range
    Dim SeriesFormula As String
    Dim ListSep As String * 1
    Dim Pos As Integer
    Dim LSeps() As Integer
    Dim Txt As String
    Dim i As Integer

    Set GetChartRange = Nothing

    On Error Resume Next
        SeriesFormula = Ch.SeriesCollection(Ser).Formula
    ListSep = ","
    For i = 1 To Len(SeriesFormula)
        If Mid$(SeriesFormula, i, 1) = ListSep Then
            Pos = Pos + 1
            ReDim Preserve LSeps(Pos)
            LSeps(Pos) = i
        End If
    Next i

    If UCase(ValXorY) = "XVALUES" Then
        Txt = Mid$(SeriesFormula, LSeps(1) + 1, LSeps(2) - LSeps(1) - 1)

        Set GetChartRange = Range(Txt)
    End If

    If UCase(ValXorY) = "VALUES" Then
        Txt = Mid$(SeriesFormula, LSeps(2) + 1, LSeps(3) - LSeps(2) - 1)

        Set GetChartRange = Range(Txt)
    End If
End Function

When you select a chart and then run the PosNegLine macro, it looks through the chart and, for line segments between negative data point values, changes the line color to red. For line segments connecting positive data points, the line color is set to green.

The problem with this solution is that it provides only an approximation; it only works with lines connecting two data points, and it can either change the entire line segment or not. If the beginning data point is positive and the ending data point is negative, it cannot change the color of a line right as it passes into negative values.

Another approach is to format data points as different colors or shapes, based on whether they are positive or negative. A way to accomplish this is detailed at Jon Peltier's Web site, located here:

http://www.peltiertech.com/Excel/Charts/ConditionalChart1.html