What is the law on proportionality under the Eighth Amendment?

"The Eighth Amendment prohibits imposition of a sentence that is 'grossly disproportionate' to the severity of the crime. [Citations.] In a noncapital case, however, successful proportionality challenges are '"exceedingly rare."' [Citation.] In the rare case where disproportionality can be inferred from (1) the gravity of the offense and harshness of the penalty, the court will consider (2) sentences imposed for other offenses in the same jurisdiction and (3) sentences imposed for commission of the same crimes in other jurisdictions. [Citation.] '[I]t is only in the rare case where a comparison of the crime committed and the sentence imposed leads to an inference of gross disproportionality that the second and third criteria come into play.' [Citations.]" (People v. Haller (2009) 174 Cal.App.4th 1080, 1087-1088.)