Edward Tufte introduced Slope Graphs in The Visual Display of Quantitative Information (p. 158):
Slopegraphs compare changes over time for a list of nouns located on an ordinal or interval scale. This table-graphic above organizes data for viewing in several directions. When read vertically, the chart ranks 15 countries by government tax collections in 1970 and again in 1979, with the names spaced in proportion to the percentages. Across the columns, the paired comparisons show how the numbers changed over the years. The slopes are also compared by reading down the collection of lines, and lines of unusual slope stand out from the overall upward pattern. The information shown is both integrated and separated: integrated through its connected content, separated in that the eye follows several different and uncluttered paths in looking over the data.
This tutorial will show how to make a slope graph in Excel.
When to Use Slope Graphs
The Economist used a pair of donut charts to show changing pre-tax profits among banks from 2007 to 2011 in Bank Profits Head East. This is not a very effective approach.
In Arrow Charts and Other Alternatives to Multiple Pie Charts on the Forbes magazine web site, Naomi Robbins introduced Arrow Charts as a replacement for double pie charts (and double donuts are at least as bad). I wrote a tutorial on my blog that showed How to Make Arrow Charts in Excel. The technique results in a much more effective chart:
Naomi showed a slope graph in her Forbes blog post. You can read more about slope graphs in Slopegraphs for comparing gradients: Slopegraph theory and practice on the bulletin board on Edward Tufte’s web site, and in Edward Tufte’s “Slopegraphs” on Charlie Park’s blog.
Making a Slope Graph
Here is the data for our slope graph. Unlike the arrow chart, we do not need to make any adjustments to the data prior to building the chart.
Select the data range (A4:C10) and insert a line chart without markers.
Excel assumes the series are in columns because there are many more rows of data. On the Chart Tools > Design tab, click on Switch Row/Column.
Hide the vertical axis by setting Major and Minor Tick Marks and Tick Labels to None, and select No Line for line color.
Format the horizontal axis, and change the Position Axis setting to On Tick marks. You might also want to use a lighter gray than the default for the axis line.
Delete the legend, make the chart taller, and format the series lines so they are thinner (1.5 pt wide rather than the default 2.25 pt). There is still plenty of white space to the left of the chart where the legend had been.
Select a series, and add data labels. here I used the default label (which is the value) in the Right position.
Select the first label (single click to select all the labels, then single click again to select the one label), and format it so it is in the Left position. Also color the label font to match the line color. For text I like to use one tile darker in the Excel color palette.
Add and format labels as above for the rest of the series.
Reposition any overlapping labels. Single click twice on a label, then you can drag it into position. The chart is wider because I stretched it to the left to fill the too-wide margin I began with.
Insert text boxes from the Chart Tools > Layout tab. Type the names of the regions, color the text to match the data labels, and position the text boxes next to the labels.
Finally, add a chart title, and the slope graph is finished.
I generally prefer my labels to the right of the chart, but a lot of these labels are clustered at the bottom of the 2011 data. It might be better to position the region labels to the left of the chart.
The slope graph shows that while most regions saw little change, Asia Pacific had a large increase and Western Europe had a similarly large decrease. The arrow chart also showed this, but it was not easy to see in the original double donut.