World population by age and region in 2002
This page both shows a graphical representation of the joint, marginal and conditional probabilities and also explains the relationship between them.
The area of each rectangle equals the joint proportion of an x-value and a y-value. Click on any rectangle — its height is P(y) and its width is P(x given y). Therefore
Joint propn = marginal propn times conditional propn
Click the box at the bottom right to alter the shape of the rectangles (but keep their areas the same). Now the width is P(x) and the height is P(y given x).
You might observe that:
The data set gives the world population in 2002, classified by age group and the region of the world.