Change width of line in 2d line chart

I have created a 2d line chart, and Numbers draws blue lines connecting my datapoints. I would like to change the width of these connecting lines, but can't seem to find anywhere to do so. The only formatting option that seems to affect these lines at all is the "Connection Lines" setting, to set them straight or curved. Can you change the thickness of this line somewhere? Can you change the color?

Clicking on the line itself only selects the data points, like this:

There are no options here for styling the line's width or color.