When you create charts within PowerPoint, typically the Chart Area doesn’t show any fill or any kind of formatting. The exception to this rule is if you have applied a particular Chart Style. Thus in most scenarios, the Chart Area is completely transparent with no fill or outline attributes. This default status works well most of the time since the Chart Area is just the area above which all chart elements are placed. However, if your slide background is crowded, then the Chart Area’s transparency can pose a problem. To counter this issue, you can fill the Chart Area with a solid color, a gradient or even a pattern to add some visual separation from the slide’s background.
Learn about the Fill and Line options for the Chart Area in PowerPoint 2013 for Windows.

