You can add a border and change the background color for a plot.
User Interface Location: Options on the left panel in PTC Mathcad Chart
Release: PTC Mathcad Prime 5.0.0.0
Benefits and Description
You can control the look and feel of the chart. Add a border to the chart, the plot area, or both, and then change the color, style, and thickness of that border. Add a fill to the background of those areas, and choose a color, color gradient, and opacity for that background. This helps to make the data stand out in your worksheet.