Area charts come in two varieties. The default type of area chart is simply a modification of a line chart. Everything from the data point on the y axis all the way to zero is shaded. In the event your numbers are negative, then the data above the line up to zero is shaded in. Each data series you have is laid upon the others. Area charts are best to use when we want to compare similar elements, for example, sales by each division in our company or revenue among product categories.

The other variation of an area chart is the stacked area chart. The chart starts off being built in the same way as a normal area chart. The first line is plotted and shaded below the line to zero. The difference occurs with the remaining lines. We simply stack them. To understand what happens, consider this analogy.

Each shaded line represents a wall built to the height given in the data series. Instead of building one wall behind another, we stack them on top of each other. What can be hard to understand is the y axis. It now denotes a cumulative total, not the individual data points.

For example, if the first y value of a line is 4 and the first y value on the second line is 5, then the second point will be plotted at 9 on our y axis. Consider this more complicated example: if the y value in our first line is 2, 7 for our second line, and 4 for the third line, then the y value for our third line will be plotted at 13. That's why we need to compare similar elements.