If you are using the code from the post you mentioned, there is code to position and size the chart on the slide. You can add the line information at the same place in the macro.
Add this code to create a border around each chart, after this line:
.Height = 289.625
.Line.Style = msoLineSingle
.Line.Weight = 4
.Line.ForeColor.RGB = RGB(0, 0, 0)
.Line.Visible = msoTrue