Keynote
Add a border and background to a chart in Keynote on Mac
You can add a border and a background color to any 2D bar, column, area, or scatter chart.
Click the chart, then in the Format
sidebar, click the Chart tab.In the Chart Options section of the sidebar, select the Border checkbox.
To add a background color to the chart or choose a style and color for the border, click the disclosure arrow next to Background & Border Style, then use the controls in that section.