The Plot Area for a chart is the layer placed behind the actual chart. This layer typically has the axes on its perimeter. Also, the Data Series and Gridlines are placed above this layer. If the fill color for both the Plot Area and the Chart Area are the same, as they often are — then you may miss the difference between both these chart elements. This default status works well. However, you can format the Plot Area separately to make it look prominent. You can change its appearance by altering fills and effects with just a few clicks.
Learn about the Plot Area for charts in PowerPoint 2011 for Mac.

