NumberFormat Property
Returns or sets the format code for the object. Returns Null if the cells in the specified range don't all have the same number format. Read/write String for all objects, except for the Range object, which is read/write Variant.
expression.NumberFormat
expression Required. An expression that returns one of the above objects.
Example
This example sets the number format for the data labels for series one.
myChart.SeriesCollection(1).DataLabels.NumberFormat = "General"