The graph shows the ISS semi-major axis.
EDIT: the blue plot represents the ISS mean radius vector minus 6371 km (just to show an approximate altitude), which is equivalent to the sami-major axis (I use "mean radius vector" to avoid confusion with the osculating semi-major axis). The mean radius vector is obtained from the TLEs downloaded from https://www.space-track.org/ and all the calculations are done with the CSpOC's SGP4 library downloaded from the same site.
There are no reboosts for an unusually long period of time and according to https://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/ISS/SVPOST.html
there will be no reboosts at least for the next 10 days.
Does anyone know the reason?