Final answer:
The horizontal axis of the H-R diagram is used to plot the temperature or spectral type of stars, with temperature increasing towards the left.
Step-by-step explanation:
The horizontal axis of the Hertzsprung-Russell diagram, or H-R diagram, is used to plot either temperature or spectral type of stars. Temperature is conventionally plotted increasing towards the left. Most stars are found on the main sequence, which spans from the upper left corner (hot, highly luminous) down to the lower right (cool, less luminous), illustrating that generally, hotter stars tend to be more luminous. This crucial part of the diagram highlights the relationship between a star's temperature and its luminosity.