Weird results out of fourier transform of unknown signal, what am I doing wrong?
My guess: nothing to see here. Move along.

You mention that "the signal appears to be a helix in [the] frequency domain". I assert that what this helix really is, is a complex exponential. And there is nothing wrong with that, because complex exponentials are what you get when you time-delay a signal: $f(x-a)\Leftrightarrow \hat{f}(\xi)e^{-2\pi i a \xi}$. The helix pattern simply reflects that most of the power in the signal you are acquiring isn't located at $T=0$. (And it shouldn't be there anyway!)

Eyeballing, the period of oscillation in your frequency plot appears to be ~28 samples. At a sampling rate of 20hz, if what I'm saying is true, then most of the energy in your time-domain signal should be centered roughly around $t\approx\frac{28}{20}=1.4$ seconds. Was I close?

The real problem here is likely to be your plots. It seems like you're plotting raw real/imag DFT outputs. Don't do that. Instead, compute magnitude/phase from real/imag values, and plot that.