The phrase "graph lines" refers to drawing or visualizing lines on a graph, usually representing the relationship between two variables. Full definition
This is also why I think anyone involved in price experiments should try enough different points to have statistically significant data to graph their line with confidence. (evilgeniuschronicles.org)
On graph after graph the lines will move in tandem. (fastgraphs.com)
You just need to get through those negative graph lines, sure enough you'll be into the positive lines soon enough. (theinspiredroom.net)