Well, is it? That depends. Sometimes yes. Sometimes no. Here is one way to find out.
If you are asking this question, you are probably familiar with the Vertical Line test to see if a graph represents a function. Once you’ve determined a graph is a function, apply a similar Horizontal Line Test to the same graph. If the horizontal line does not “touch” the graph in more than one point, then you can be assured the inverse is also a function.
Let’s leave it at that for now. Questions? More about functions coming soon.