I don't think there is a built-in way to do this, but there are a couple of hacks:
1) Use dashes. So you can type something like this:
2) Use a text box to create a line. Start by dragging a floating text box onto the dashboard. Type no text into it. Go to the layout panel and set the background color to black. Then, also in the layout panel, change the height to 1 pixel.
Thanks again Ken. That tip worked well.