Mixed Metaphor: Line Graph + Bar Chart

Many charting tools allow us to combine bar charts and line graphs in the same graphic – but should we?  This question came up when I did a double take the other day while reading a market research report.  In order to focus on function rather than content, I have re-created a similar graph below:

Click the image to enlarge it

If the author’s intent was to induce a double take, it worked for me – in a confusing sort of way:

  • It caught my eye that the line graph is trending down while the bars are going up
  • But it took a few moments to realize what the line graph actually represents
  • And it took a few more moments to realize the message behind the graph: Widget volume is going up, but the rate of growth has turned from “healthy” to “anemic”

That was way too much work to get the gist of this graph!  Why was this so difficult to get?

Instead of providing a scale, the individual data points are labeled – this clutters up the picture and distracts the eye.  Not only that, it is difficult to get a ballpark idea for the maximum values unless one scans each data point – a scale for each data series would have provided that contextual information much more quickly.

When I re-created the graph I realized something else: in order to display the line graph within the colored bars, I had to set the maximum value for the percentage scale higher than what would be expected: instead of setting the top percentage value to 10% I had to use 15% to push the line graph down far enough to completely overlap with the bars.

Also, fewer words in the legend labels would have been better (the original had even more text!) – and it would have helped to spell out the word “percent” instead of hiding the percent symbol amongst all that text.  Better yet: be consistent and stick with the same nomenclature: the chart title says “Volume and Growth” so the series labels should say the same.

Maybe this is a nit pick, but I prefer legends at the bottom or the side of a graph.  The top  should be reserved for the title – IMHO.

Finally, the question of whether we should mix bar charts with line graphs: I don’t think so.  The bars overpower the picture and clutter things up.  Call me a minimalist, but it’s much easier to see how volume and rate of growth relate to each other when we draw two clean lines like so:

Click the image to enlarge it

With less clutter, the eye can very quickly take in the overall picture and focus on what matters:

  • Number of Widgets is going up,
  • Percent Growth is going down.

If the data values really matter to us, we can look them up in the table below the graph.

3 comments to Mixed Metaphor: Line Graph + Bar Chart