How to change the border width of one series on the chart

I was able to plot three series on a chart.

I would like to change the width of one of the three series to highlight it.  

Whenever I change the border width, the same border width applies to all three series.  

I see that there is a way to apply expression to the border width.  Can someone please show me how to make only one of the three series bolder, while leaving the other two series the same width as before?

Thanks 

Henry