I wrote the following code which solves the problem for us.

Assign the following code to a keypress, select the chart to update and run
the code.

Sub ChartFormatting()
'Written 15 Oct 2007
'Routine to reformat graphs in Excel 2007 to set positive value to be colour
1 i.e. blue and negative values to be colour 2 i.e. red

'Declare variables
Dim i As Integer
Dim DataPoints As Integer
Dim X As Object
Dim XValuesArray
Dim ChartCheck As Boolean
Dim PositiveFill, NegativeFill As Integer
Dim ChartType1, ChartType2 As Integer
   
'Set Values
PositiveFill = 32    'i.e. blue fill for positives
NegativeFill = 3     'i.e. Red fill for negatives

ChartType1 = 51      'i.e. a basic column chart
ChartType2 = 57      'i.e. a basic bar chart

'Check if we have a chart selected, calls ChartIsSelected function
If Not ChartIsSelected Then
   Exit Sub
End If

' Calculate the number of data points.
DataPoints = UBound(ActiveChart.SeriesCollection(1).Values)

For Each X In ActiveChart.SeriesCollection

   'Set array size to contain graph point values
   ReDim XValuesArray(1, DataPoints)
   'Read in values from graph and store in array
   XValuesArray = X.Values
   
   'Checks the chart type to see if this is a basic bar chart or column
chart series, otherwise ignores
   ChartCheck = X.ChartType = ChartType1 Or X.ChartType = ChartType2
   If ChartCheck Then
       'For each value in chart series
       For i = 1 To UBound(XValuesArray)
           'Format if positive value
           If XValuesArray(i) >= 0 Then
               X.Points(i).Select
               With Selection.Border
                   .Weight = xlThin
                   .LineStyle = xlNone
               End With
               Selection.Shadow = False
               Selection.InvertIfNegative = False
               With Selection.Interior
                   .ColorIndex = PositiveFill
                   .Pattern = xlSolid
               End With
           Else
           'Format if negative value
               X.Points(i).Select
               With Selection.Border
                   .Weight = xlThin
                   .LineStyle = xlNone
               End With
               Selection.Shadow = False
               Selection.InvertIfNegative = False
               With Selection.Interior
                   .ColorIndex = NegativeFill
                   .Pattern = xlSolid
               End With
           End If
       Next i
   End If
   'Deselect chart
   ActiveChart.Deselect
Next
 
End Sub

Private Function ChartIsSelected() As Boolean
'ChartIsSelected function, returns true if we have selected a chart or
embedded chart option, otherwise false
   ChartIsSelected = Not ActiveChart Is Nothing
End Function

Hopefully this does what everyone is looking for.

Cheers

Gareth