A good presentation uses very simple data charts. Often, in the last minute, changes are made to the data: it is easy to change a number in PowerPoint here and there.
In most cases the first versions of these simple data charts were extracted from very complicated Excel sheets. And here is where the trouble starts. After a few PowerPoint iterations, the presentation and the backup model is no longer consistent. This is fine if the presentation was a one-off event, but most of the time, the Excel model will live on for future iterations.
The solution is to include a worksheet in your Excel model that pulls the data exactly as it goes in your charts. Put in the correct rounding, everything. Anyone who wants to change the numbers, need to make changes in the Excel model to get the numbers to change.
And yes, sometimes that might involve a goal seek.
If these charts frequently change, you might even consider designing a presentation in Excel. Excel has the same chart and shape design tools as PowerPoint, and you can create direct links between charts and work sheets without having to copy things across. See a previous post (2009) on this technique.