You have some data to include in a report; should it go in as a table, a graph or both? Research shows that tables are best suited for looking up specific information, and graphs are better for perceiving trends and making comparisons and predictions. In this post, I’m looking at ‘slopegraphs’ which provide a simple way of comparing results between groups.
Let’s look at some hydrologic examples. The Bureau of Meteorology recently published the National Performance Report 2014-15 for Urban Water Utilities. It contains lots of tables that compare values for Australia’s main cities. An example is shown below which shows customer complaints related to water and sewerage. If you’d like to know how many complaints there were in Perth in 2010-11, per 1,000 properties, then this table has the data you need.
Now check out the same data as a slopegraph (Figure 1). It is certainly much easier to see trends and make comparisons. Clearly Darwin dominates when it comes to complaints; plus we can see that Perth has done well to reduce the number of complaints over the last 5 years.
Another example is capital expenditure by water utilities (Table below). If you need the actual dollar amounts, the table is the place to look. However, the graph is much better at showing the decline in expenditure since 2011-12 (Figure 2). This is even more obvious if we include the total expenditure per year (Figure 3).
Slopegraphs (also known as ‘bump charts’ or ‘parallel coordinate plots’) are very useful for displaying the type of data that is in these tables. They were popularised by Edward Tufte is his book, Beautiful Evidence and use a minimum of ink to display information. The coloured lines make it easy to distinguish cities and the labels at both ends of the lines means a legend is not required.
Code to reproduce the figures in this blog is available as a gist. Hints for making these graphs in Excel are available here. As at 5 May 2017, the slopegraph functions in Thomas Leepers package were upgraded.