proportional editing along one axis only?

I want to proportionally scale a double helix along it’s length so that the “wavelength” widens and shortens. This would involve proportional scaling along one axis, but is this possible? Or is there another way? I need to be able to translate in the same way also.

Try constraining to one axis

S X constrains scale to X axis
S Y constrains scale to Y axis
S Z constrains scale to Z axis