We see that on this plot most stars fall very near a straight line. This is called the mass-luminosity relation for main-sequence stars.
The adjacent plot implies a very strong dependence of the luminosity on the mass, since the mass enters raised to the power 3.5. For example, if I double the mass of a main sequence star, the luminosity increases by a factor 2 3.5 ~ 11.3. Thus, stars like Sirius that are about twice as massive as the Sun are more than 10 times as luminous.
The observation of a correlation between mass and luminosity for particular classes of stars suggests important systematics relating the light output of stars to their intrinsic structure that will later be exploited in the Hertzsprung-Russell Diagram.