Straight lines share the generic equation:
y=mx+b
essentially, you can say that straight lines signify only proportional quantities, like a=b or a=(-b)
T=1/f is not a parabola, that needs to be understood now. A parabola is (essentially) in the form y=ax^n where n is any whole integer. Parabolas look like a U.
T=1/f is an inversely proportional relationship- and graphed in the form T=1/f, f can never be zero (or T will be infinity), which means T can never be 0 (since no value of f except infinity can make it zero). This is not how a line works, since any line would pass through 0 for one of the variables.
To answer your question in a sentence: Inversely proportional relationships never look like lines, because they can never have the generic form y=mx+b.
