The phrase "yellow line" usually refers to a painted line on the road that tells drivers where they can and cannot park their vehicles. Full definition
You can see it as a bottom yellow line on the chart below. (earnforex.com)
Yellow lines show individual results from different climate models, and the black line shows observed temperatures to the present day. (carbonbrief.org)
4 — You're going 45 miles per hour on a two - lane, rural road with a double yellow line in the center. (myimprov.com)