Gridlines Object (PowerPoint)
Represents major or minor gridlines on a chart axis.
Gridlines extend the tick marks on a chart axis to make it easier to see the values associated with the data markers. This object is not a collection. There is no object that represents a single gridline; you either enable all gridlines for an axis or disable all of them.
Use the MajorGridlines property to return the GridLines object that represents the major gridlines for the axis. Use the MinorGridlines property to return the GridLines object that represents the minor gridlines. It is possible to return both major and minor gridlines at the same time.
Although the following code applies to Microsoft Word, you can readily modify it to apply to PowerPoint.
The following example enables major gridlines for the category axis of the first chart in the active document and then formats the gridlines to be blue dashed lines.