The plot is logarithmic on all three axes.
The microwave band by one convention is redward of 0.1 cm; blueward is infrared (HZ-54).
The data is excellently fit by a blackbody spectrum CMB T = 2.72548(57) K (Fixsen 2009) (see also Wikipedia: Cosmic microwave background radiation: Features).
Credit: CMB Astrophysics Research Program: COBE site This is a Lawrence Berkeley Lab site reporting on George Smoot's group.
Why is there no scale?
The average temperature is 2.725+/-0.001 K.
The colors code deviations in micro-Kelvins: dark blue (-200), green (0), red (+200).
Credit: NASA/WMAP Science Team.
But rather vaguely, the top plot shows of frequency of hot and cold spots of various angular sizes on the sky (FK-652--653). Large spots are on the left; small spots on the right.
The data points come from the WMAP satellite and other detectors.
The curve is fit to the data???---but with how many free parameters---of what is called a scale-invariant power spectrum which is a prediction of most inflation models irregardless of the details of the inflation mechanism (CL-263,265,274--275; Gr-309).
The fact that inflation predicted the observations is an astonishing triumph of the inflation idea.
It does NOT prove inflation, but it certainly strongly supports it.
Credit: NASA/WMAP Science Team.