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snowflakeuniverse: (As another example of Astronomers confusing way to present data, they draw the Hertzsprung-Russell diagram partially “backwards”. The Y-axis is usually associated with the energy output, and as one reads vertically up the y-axis, luminosity increases. This is great. The X-axis on the other hand has decreasing temperature measures as one reads further out horizontally.)
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But it measures increasing wavelength of peak energy output as you move from left to right, so it depends upon which factor you're talking about. At any rate, if you look at the original plots of Henry Norris Russell, you'll see that what he plotted was spectral class on the X-axis going from B on the left to M on the right. (There actually was a spot for an "N" class on the one I'm staring at but no stars were plotted.) It so happens that B stars are hotter than M stars. Russell was aware of this. One of the things his graph showed was that there were cool M-class stars with very large absolute magnitudes. In the luminosity formula, temperature and radius are your variables which meant that the only way to have a really high absolute magnitude star of a cool temperature was if the star had a large radius.
The H-R diagram does offer astronomers a method of interpreting the evolutionary history of stars and star clusters. Open and Globular star clusters can be age dated from the "main sequence turnoff". Since O-class stars last very short amount of time they're the first ones to evolve off the main sequence - and then the stars in the cluster continue to peel off from the top down as the cluster ages.