rectilinear, rectilineal, collinear, lengthwise, lengthways, analog, bilinear, additive, one-dimensional, analogue, more...

nonlinear, circular, zigzag, meandering, convoluted

straight, curved, parabolic, hyperbolic, elliptical, circular, spherical, cylindrical, rectangular, more...

non, more, piecewise, simple, less, log, longer, best, only, narrow, wise

in a straight line,   like tau or s,   straight direction,   one direction,   like an arrow,   one dimentional,   kind of function,   long and straight,   arranged in rows,   of rows