Yes, the function square(x) returns a unique value, either +1 or -1, for each vallue of x. There is no value of x for which it returns both +1 and -1, so when you plot sin(x) vs (x), there is no value of x for which the plot will produce a line that is exactly vertical. In order to produce such a graph, you need to generate a data set in which there are two identical x-values at each transition, with one of them having a corresponding y-value of -1 and the other having a y-value of +1.