Doktor Hertzsprung, Doctor Russell,
Why is your diagram such a tussle ?
Why is your axis the wrong way round,
Higher to lower, bluer to redder ?
Does it reveal some secret profound,
Hotter to cooler, younger to deader ?
Your sweeping main sequence is mirrored about,
Your plot’s lost the plot, and your graph’s up the spout.
Actually, I’m being a little unfair here. The original plot was absolute magnitude (aka luminosity) on the y-axis, and the peak wavelength of light on the x (or ‘spectral type’). Since high-energy blue light has many more waves in the same distance as lower-energy red light, so the length between peaks in red light is longer, and thus a higher number. (Incidentally, Omicron2 Eriadne B was the first white dwarf discovered in 1910.)
When it was later realised that the wavelengths directly related to surface tempurature, the latter was substituted, but now the numbers were going down instead of up, and nobody bothered to flip it. But thanks to the wonders of the internet, I was able to rustle up the following: