I tried to use planets' HeliocentricLongitude and I wonder if anyone has any insight on what coordinate system they use. I took vernal equinox times from http://ns1763.ca/equinox/eqindex.html and calculated Earth's longitude (minus 180°). That's what it looks like (there is a margin of error of ~2.5'' due to nearest-minute rounding of equinox times)
The 18-years cycle looks like the effect of nutation, which makes me wonder - do they account for this effect when calculating equinox times (and so Wolfram's coordinates just rotate with the Point of Aries at 1°/72 years) or is it Wolfram that accounts for nutation adjusting its coordinate system accordingly? And shouldn't it be centered? And what's this downslope beyond 2000?