A chart object in a Microsoft PowerPoint presentation is changed programmatically by using the object model. However, if the presentation is saved, another user of the presentation cannot detect changes that were made during the co-authoring session. Therefore, the changes are not merged into the user's presentation.
To work around this problem, select the chart object programmatically. Then, change the chart object.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.