Simple change of chart starting point creates misleading impression

Graphical presentation depicts quantitative data effectively. It can help readers build up a clearer impression of the trend and distribution of the data than columns of figures can.

However, if not done properly, it can easily cause an optical illusion and mislead readers.

Look at the two bar charts (Diagrams 1 and 2) presented above. Both diagrams present the sales turnover of a company from 1995 to 1998.

A quick glance at Diagram 1 may give readers the wrong impression that the turnover in 1998 was double that in 1997 and four times that in 1995. This is because the bar depicting the turnover in 1998 is two times as high as that in 1997 and four times as high as that in 1995.

On the contrary, casual viewers of Diagram 2 may think that there had not been any notable changes in turnover from 1995 to 1998 since the heights of the bars look almost the same (particularly those depicting turnover from 1995 to 1997).

What causes the differences in the messages perceived by readers of these two diagrams? It is the initial value of the y-axes of the bar charts. In Diagram 1, a 'non-zero' starting point is used while in Diagram 2, a 'zero' starting point is used.

Because the initial value of the y-axis in Diagram 1 is set at $1,000 million, all the relative differences in turnover (for turnover above $1,000 million) have been shown in the bar chart in an exaggerated manner.