Could you be more explicit about what you mean by a vertical reference line?

It's really self-explanatory... Vertical is perpendicular to the horizontal; at right angles to the horizon. A reference line is a line that shows a point of reference.

In relation to a graph, a vertical reference line is a line that extends from the top of the graph to the bottom of the graph at a point on the X axis to give a frame of reference for the rest of the graph.