You can always undo the changes in PowerPoint. Close the datasheet to return to PowerPoint and click the Undo button on the Quick Access Toolbar (or press Ctrl+Z ). The chart on the slide will revert to its previous state, and the changes in the chart's datasheet will be undone accordingly.
think-cell's datasheet is an Excel sheet. The datasheet is saved whenever the PowerPoint slide is updated after you changed some data. Unfortunately, Excel always discards its undo-buffer on save. You can experience the same behavior when working directly with Excel after saving the workbook.