The (original) Hubble Diagram was a graph created by Edwin Hubble to demonstrate that galaxies are moving away from each other in a certain pattern, created by graphing the Redshift of galaxies against their distance, for a set of galaxies whose distance had been determined. It demonstrated that the rate of spreading was constant among the galaxies, now known as Hubble's Law, and a measure of the rate being the Hubble Constant.
Hubble diagrams are still created using newer techniques and accuracy in determining the distances to galaxies, such as the use of Type Ia Supernova Standard Candles. Current interest is not only to refine the measurement of the Hubble constant but to look for variation, e.g., the study that brought Dark Energy to light.
Hubble Constant (H0)
Supernova Cosmology Project (SCP)