If I draw this down this this point is this year's income down here and this point here is next year's income; I'll use Y for income.

过这点画一条x轴的垂线,表示今年的收入,再画一条y轴的垂线,表示下一年的收入,我用y表示收入

耶鲁公开课 - 金融市场课程节选