Perspective is a key concern in graphs, as in other visual arts. Mixing multiple perspectives on the same chart can be disorienting, as in this recent WSJ chart depicting the sharp drop in stock market values across the world in May 2006:
Despite its complicated appearance, the entire graph can be constructed using only 20 numbers (the stock market value in May and in June for each of the 10 countries). The loss in value is offered in three perspectives: as a percentage (hanging bar); as a number (list); and as a change in square area (inverted L-shaped bit of the square). Needless to say, such redundancy only serves to confuse readers. More harmful is the square representation but I'll leave that for another day.
The message can be more clearly conveyed by focusing on one and only one perspective. Our first reconstruction stresses the absolute value lost in dollars rather than the percentage. From this perspective, the drop in US stock prices was the most severe, despite the lower percentage. It is also evident how much larger is the US market cap than the rest of the world. Consequently, the plunge in India had little impact on the global scene.
Alternatively, if our key message is the relative drop in stock market values, then we should take a different perspective. Here, I merely re-created the above chart with a log scale for the vertical axis. Based on the slopes of the line segments, one figures that India and Brazil suffered much more than the US on a percentage basis. Notice that from this perspective, the size of the US market cap does not jump out at the reader.
As preparatory work, the graphic designer should always test different perspectives and pick one that brings out her key point.
Reference: "As Inflation Signs Grow, Divergent Reactions", Wall Street Journal, June 15, 2006.
PS. Posting frequency will drop or vanish in the next two weeks depending on my vacation.