Misusing Bar Graphs

Today I noticed this tweet

The referenced picture is the following one:


Yet another example of bad visualization. As with other examples I wonder if it is just a case of visual illiteracy or a malicious use of visualization.

The graph has a series of problems:

Due to non-zero base bars, it has a large lie factor (2.2) obtained by comparing:

  • ratio of real values: 87.8 : 61.9
  • ratio on graph: 37.8 : 11.9

The lie factor is due to the non-zero base of the bars.

It uses a dual scale that confuses and makes very hard a visual comparison of the values and further distorting the compared values.

The dual scale is not mentioned anywhere and it is not clear which values refer to which scale.

In general the usage of bars is not the most appropriate visual representation if the goal is to show a trend or evolution in time. Points and lines are a much better choice.




Bar vs Dot Charts (in Excel)

I often observe (on the web or in my students works) bar charts used inappropriately. What often happens is that bar charts are used with a non-zero baseline, which implies telling a visual lie.

Here I report a very simple example of how the issue emerges when drawing a chart in Excel and a possible solution (using a dot chart).

Disclaimer: this is quick post, apologies for typos and lack of clarity.

Continua a leggere