Ok, here is the diagram, from the website Science. Note, nobody is really calling this Planet X. Brown and company are calling it Planet 9 till it's found and properly named. First, orient yourself. The orbit of Neptune is inside that tiny, bitsy little blue ring at the middle of the diagram. The planets with the wide, elliptical orbits skewed to the left are small objects (smaller than Pluto) in the Scattered Disk. That is the region beyond the Kuiper Belt, home to things like Pluto and 1992 QB1 (Eris is much further out as well, but not shown in this illustration). Scattered Disk Objects (SDOs) are called that because astronomers think they got scattered by something. They are in weird orbits that can't be explained any other way, so far. Brown and company noticed that there were several SDOs skewed in one direction -- and inferred that it was due to the presence of something much larger. They think that large thing, presumably the remnant core of a long-gone gas giant such as Jupiter, is now going to be found in the other direction -- the red wedge. See larger diagram here.
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