To check if a graph is a function, we use the vertical line test. If your run a vertical line over the function, are there any places where the line intersects the graph more than once? If so, it's not a function.
In this case, if we have a line at x = 0, the line will go through more than one point - (0, 0) and (0, 3.1) [exact value doesn't matter, what matters is that there's a point there]. That means that this is not a function, so the statement is false.
