The Disease Graph is a data chart that is available to view when one completes the game (whenever the game ends).
It shows the levels of Infectivity, Severity, and Lethality throughout the game in graph form. This graph is unlike the bars showing these disease traits in the Symptoms menu, as the symptom menu bars only show the current Infectivity, Severity, and Lethality of your disease.
The pink line is Infectivity, the yellow Severity, and the purple Lethality. It is worth mentioning that there is no Lethality in the Christmas-themed Neurax Worm scenario Santa's Little Helper, and the Severity is replaced by Happiness.