In a chart, the Plot Area is the backdrop upon which other chart elements such as Data Series and Gridlines are placed. This layer has axes on its perimeter. By default, the Plot Area has no fill attributes, and is transparent. This may look plain, and that by itself is no reason to change the fill of the Plot Area. However, there are times when the slide background itself is busy — and adding a different fill to the Plot Area can then make the chart appear more clear.