You can apply any fill to your chart’s Plot Area. Typically, the Plot Area possesses no fill attributes, and thus shows the slide’s background. The reason why you would like to change this attribute is to highlight the Data Series and Gridlines that populate your chart’s Plot Area, especially if you have a busy slide background. There are various fills that you can apply to the Plot Area, such as a solid color, a gradient, a picture, a texture, or a pattern. In this tutorial, let us learn how to apply a solid fill to the chart’s Plot Area.
Learn how to apply solid color fills to the Plot Area of a chart in PowerPoint 2011 for Mac.

