for a variable to be proportional to another, it implies a relationship of
A = kB (or B = pA)
where A and B are the related variables, and k or p is the constant depending on how you write the equation. thus they can be related as a straight linear line.
the radius does change when the period changes, but that is more of a statement like 'the radius increases when the period is longer', note that this statement does not really have any quantifiable mathematical relationship.