Nyquist Frequency on semi-unevenly sampled data
the lomb scargle in matlab, (plomb) returns a frequency vector as the second output. i would be surprised if the routine you are using doesn’t do the same.  there is a common tendency for python signal processing libraries to be functionally equivalent to matlab conventions.

your plots show symmetry around a “center” frequency which is the same as what a FFT does for real inputs, except a FFT would not extend to start a third interval.  The high half frequencies(above $\approx$ .017 to .033)  are the negative frequencies in FFT speak. you can ignore them when you have a real signal. 

There really isn’t a simple “Nyquist” frequency with nonuniform sampling. 

The documentation of the library you are using should detail how frequency is treated. If it doesn’t, you can find a routine that does or do your own calibration by using test sines of known frequency.