Rank and age in a university
Use this diagram to explain that the marginal counts for X and Y are the marginal totals of a contingency table, and that they therefore correspond to a sum of bar heights in a clustered bar chart.
Click Stacked to add up the frequencies for each age group. The marginal distribution for Age is given by the stacks of bars.
Unstack, select Rank from the pop-up menu, then stack again to do the corresponding stacking to find the marginal distribution of Rank.
The data are taken from a textbook but are probably artificial.