The platform of a chart is the container of the other chart elements, such as the paper, legend, and labels.
To format the platform of a chart:
Click the color image button and select a color swatch to fill the platform, and specify the transparency of the color schema (you can also directly type the hexadecimal value of the desired color in the text box). If the color swatches cannot meet your requirement, click More Colors on the color palette to open the Pick a Color dialog, with which you can set a full range of color, or click More Fill Effects to specify the fill effect in the Fill Effects dialog.
See also Format Platform dialog for detailed explanation about options in the dialog.