Oh yes- look at the differential time scales of the two graphs. The dips in ISS altitude have a solar/terrestrial component - but there is a human component as well. They let it drop due to drag - but also for ops reasons.

NASA/Russia allowed ISS to drop to a low altitude early on for several reasons: a long gap before Shuttle began ISS assembly in earnest. Early on ISS was only FGB and then Node I for a long time - the delay was due to more than a year's delay in Service Module completion and launch. FGB had a limited prop capability so they stretched it out as long as they could - hence orbit dips. Indeed it exceeded design life before SM was launched.

Pacing of dips also syhchs up with either Shuttle orbiter or Progress visits - and reboosts as well.

Also, allowing orbit dips increased ease for Shuttle to reach a lower orbiting ISS - with more payload.