This is the very worst diagram I have seen in a scientific paper. What I have reproduced here is exactly as it is in the paper.
They started with a program that produced a graph as lines, then dumped the image to a file as a black and white bitmap of low resolution. So it is completely impossible to read the axes labels for the file inset, and the main diagram is not much better.
Furthermore, the units /K are meaningless. They mean the inverse of K, but merely putting in a slash doesn’t capture that.
I find it hard to understand why the referees and editors failed to pick this up. The journal itself (Physical Review B) is sufficiently popular that it has become increasingly hard to publish in.
If I worked with someone who produced a graph like this, they would get a reaction from me that would ensure they would never produce such rubbish again.
The culprit paper is entitled “Anomalous dynamical charges, phonons, and the origin of negative thermal expansion in Y2W3O12”, and it is published in volume 76 (year 2007) of Physical Review B, with article number 024307 (you can find it here). In my view there are scientific problems with the paper, but I will save my criticisms for off-line if anyone is interested.