When series data is stacked on a percent axis, all the values in each category are added together. That sum is considered 100 percent. Each data point in the category is drawn as a percentage of that sum. You can stack series on the Order tab of the Series menu and the set the Y-axis scale to a percent on the Scale Type tab of the Value Axis menu.

The following illustration shows the same chart with regular stacking and stacking on a percent axis.