A proportional 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