During an outbreak investigation, plotting the number of cases by their date of onset helps to visualize the epidemic's course. This graphical representation is known as a/an:

Explanation

An epidemic curve is a histogram that shows the number of cases of a disease by the date of onset. It is a crucial tool in outbreak investigation as it helps to visualize the magnitude, pattern (e.g., common source, propagated), and time course of an epidemic, providing clues about the source and mode of transmission.