Showing two main points on one chart

It’s (usually) fairly straightforward to choose a chart type when you know what the main point you are trying to get across is. Is your message that there has been a change over time? Do you want to show a difference between groups? There are all kinds of online chart choosers to help you do this (here is one of my favourites). But what about when you have two main points to make?

I was recently working on a chart where I wanted to make the following two points:

  1. 2016 was the only year that participants had a statistically signifcant increase in health ratings; and
  2. participants had lower health ratings pre-program in 2016 vs. other years

I started with the chart below. Here the different color used in 2016 really highlights that something different happend that year (half of point #2), but it is difficult to see the change over time (point #1, half of point #2):

chart1

Alright then, let’s change to a line graph. It is much easier to see the change over time. However, the statistical change in pre- and post-test scores was important to the program and they wanted to highlight that. That piece of information isn’t easy to see here.

chart2

I added a transparent rectangle to highlight the difference between pre- and post-test scores and this is the result:

chart3

I think that this chart nicely conveys the two main points that I wanted to make and is a vast improvement over the first chart. It also goes to show that it’s worthwhile to play around with different chart types while working on reporting!

Note: I have changed the results to fictional data to keep things anonymous