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

Where m is the slope of the line and b is the y-intercept of the line, or the y-coordinate of the point at which the line crosses the y-axis. to write an equation in slope-intercept form, given a graph of that equation, pick two points on the line and use them to find the slope. this is the value of m in the equation.