If you are working with Charts programmatically in Excel 2007 Service Pack 2, and are trying to set the formatting of individual characters of Data Labels of charts programmatically, you will notice that the formatting gets applied to the whole of Data Label instead of the specific character/range.
Here is a snippet which one would typically be used to set formatting of specific characters:
Dim ch As Chart
Dim series As series
Dim points As points
Dim label As DataLabel
Dim chars As Characters
Dim font As font
Set ch = ActiveSheet.Shapes(1).Chart
Set series = ch.SeriesCollection(1)
Set points = series.points
Set label = points(1).DataLabel
label.Text = label.Text & vbCrLf & "a"
Set chars = label.Characters(3, 1)
Set font = chars.font
font.Size = 24
The above code will work as desired (format individual characters) on Excel 2003 and Excel 2007 Service Pack 1 releases.
This behavior is a known bug with Excel 2007 Service Pack 2 release, and the Excel development team is looking into it. Unfortunately, there is no known programmatic workaround possible for this requirement. Character level formatting of Data labels can only be set manually as of now.