Add a border and background to a chart in Numbers 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 triangle next to Background & Border Style, then use the controls in that section.
You can also add a border around your sheet. To learn more, see Add a border around a sheet.