This could be the luminosity in watts. More commonly it is in units of the Sun's luminosity. Absolute magnitude is also commonly used. The horizontal position represents the star's surface temperature. Sometimes this is labelled in by the temperature in Kelvins.
Highest temperatures go to the left.
One could also use a measure of color as seen through filters. The Hertzsprung-Russell diagram us an example of a scatter plot. Such plots are useful outside of astronomy too. What one sees in the HR diagram When data on the nearest stars to us or stars in a cluster are plotted in a HR diagram, here is what one sees . We expect a relation between temperature and luminosity .