By changing the series order, you can manipulate the set of data the viewers' eyes will see first. Why do this? It's really a form of advertising—if you want to get a point across without getting lost in a conversation about all of the other elements, you place the data in the most viewable place within a chart, which is the first series. Because people generally look at information from left to right or top to bottom, this will naturally allow you to flow into a discussion about which data set is most important in the chart. There are ways to draw focus using formatting that far outweigh series order, but series order is an important tool to gain control over the chart and the audience.
Reversing the data series is a trick that comes in handy from time to time, particularly with bar charts. For example, if you create a bar chart with quarters, Excel plots the last series of quarters at the top (see Figure 14.23). Because we naturally view information in a top-down manner, you'll probably want to reverse the order in which the series are viewed, so that Q1 appears at the top of the chart and Q4 at the bottom (see Figure 14.24).