What is the variable that is always indicated by the "y" axis when graphed?

Generally, the independent variable is graphed on the x axis and the dependent variable is graphed on the y axis. 

However, bear in mind that this is only a convention (a generally accepted practice) and not a hard and fast rule. Particularly on exams and standardized tests, the test writer may interchange the variables on the axes in an effort to confuse the test taker. You should always take a moment to identify which quantity is graphed on each axis and whether that quantity represents the independent or the dependent variable in the situation.

Remember that the independent variable is the one which the experimenter manipulates or controls, and the dependent variable is the one which changes as a result of the experimenter's changes to the independent variable.