Ways to indicate a series is selected on a line chart

I have a program that displays a dense collection of line series. I can select series from a tree view to the left of the chart.

My issue is I need an elegant way to show the user which line series is selected. Thicker lines dont work as they tend to get in the way of other lines. Dashed lines dont work as the the lines are so volatile you either miss out on important features of the graph, or cant tell its dashed at all.

Any other solutions?