smc

$1$ goldfish costs $1\cdot 15=15$ cents, $2$ goldfish cost $2\cdot 15=30$ cents, and so on until $12$ fish. One may think that it is a straight line segment at first, but note that there cannot be a third of a goldfish sold (if so then that would be creepy). Thus our answer is $\boxed{\text{a set of horizontal parallel line segments}}$.