The vertical line test can be used to determine whether a graph represents a function. ... If we can draw any vertical line that intersects a graph more than once, then the graph does not define a function because that x value has more than one output. A function has only one output value for each input value.
None of the dots pass threw one another when doing the line test for the x value, but when doing the line test two y values pass threw one another. So it's one-to-many
