Most definitely there can be several lines.
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Draw a vertical line if the line hits more than one point on the graph then it is not a function.
In simple terms, if a vertical line meets a graph in more than one place it is not a function.
If a vertical line intersects the graph at more than one point then it is not a function.
A graph is more informative than an equation because a graph is easier to interpret visually, and find all the points and line them up, rather than just a slope which shows no points(data).
No vertical line will intersect the graph in more than one point. The fundamental flaw is that no graph can show that it does not happen beyond the domain of the graph.