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.

In reply to Re: Re: Re: Re: Drawing reference lines with GD::Graph by Mr. Muskrat
in thread Drawing reference lines with GD::Graph by phildog

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