Aproportional relationship must graph as a line, is that false?

A direct proportional relation, could be linear, like y = 3x, in which case you get a line, however it could also be exponential or even logarithmic or radical, in which cases, you do not get a line, but just a sinusoid graph.

y = 3x²  <--- parabolic graph

y = 3√x  <--- parabolic as well