Line Graphs - some data appears with 2 colors

What could cause one dataset in a line graph to show up with two colors? I have a graph with 3 lines, and the third one has a faint orange doubling the main line.

I've been selecting the data but don't see anything to indicate a superposition. However, at some point in my exploration, I found that the marker showed orange - but can't figure out what is going on.

I would really appreciate any insights, thank you!