In PowerPoint 2007 and later, a dialog appears when you deactivate think-cell and try to manually edit charts created with think-cell. The image below shows PowerPoint 2013 - the exact text differs slightly between PowerPoint versions:
PowerPoint 2007 and later use Excel's charting engine by default and offer to convert Microsoft Graph charts into Excel charts. Most of think-cell's charts are based on Microsoft Graph, as were all charts in older versions of PowerPoint up to version 2003.
Choose Edit existing in the above dialog. Do not choose to Convert think-cell charts, as this will needlessly trigger a reimport. You can configure PowerPoint 2007 and later to not show the above dialog and to return to the behavior of older PowerPoint versions.
To disable the above dialog, add the following key (DWORD) to the registry and set its value to 1:
Note: PowerPoint will also insert a Microsoft Graph chart instead of an Excel chart when clicking PowerPoint's built-in Chart button.