We can use the Shape Outline options to apply a border to an Excel chart. We use the following steps: Click just below the top boundary of the chart to select the Chart Area of the graph. When the chart is selected, the contextual Format tab on the Ribbon is activated.

You can change the color, width, and line style of lines in a chart (such as gridlines, axes, trendlines, or error bars), or you can create custom borders for selected shapes (such as data markers, titles, data labels, or a legend). To remove the outline color from the selected chart element, click No Outline