I'm a bit more ecumenical in my OSes (Bill has had me over to Seattle twice, so I will always be a loss maker for him), I run many different ones.
It's badged as a HLog graph as shown by the tool but under the covers it has three outputs - hybrid response data (4096 points), Impulse Response Data (8192 points) and Phasese Response Data (4096 points between +/- 3.125... I assume pi for now).
It was the manic bit piqued my interest. There are clearly 2 sinusoids in the graph; one more noticeable in the low tones which attenuates quickly, but is present throughout, and one you've picked up by the symmetry. I tried to autocorrelate, but failed in the low tones, then couldn't get a good fit for a sine wave as there were two interacting (but I feel if I reverse out the 27 and 250 using a sine function with some attenuation built in, I learn something more) - so looked at all the minima and maxima.
There were two clear periods in the data one of 27 tones and one about 250 tones, as well as extra fun caused by the interactions. I tinkered up some code to measure across 4 avoiding noise, to give slightly more accuracy, and then looked up the distance needed for that reflection.
- that nearly took more typing than the code.
Edit: forgot to state the obvious. The shortest period sinusoid should always be the line length (as that's the furthest thing measured) and if there is a second (or third) sinusoid - it'll be between the start point and the line end (hence why I calculated from each end of the frequency range, as the furthest response should attenuate quicker, leaving any other reflection visible).