I often see bar charts used to display changes over time, or “before and after” comparisons. While this is a perfectly fine chart to use, there are much more effective and creative ways to display changes between two time points. One of my favourite ways is to use small multiple slope charts.

The following vertical bar came across my desk a while back. It looks at pre and post course evaluations. It was clearly done in Excel as we can see from the lovely (hideous) colour palette it spat out. On first glance, we can see the overall differences between the red and blue categories (pre and post) but actual values are a little more difficult to determine, as there are no guiding grid lines. Our eyes also dart back and forth from the bars to the key to match the categories.

Excel bar chart showing before, after and difference bars.

The interesting part about this chart too, is that there has been a third category added which is looking at the difference between pre and post (the first two bars). The inclusion of this third bar is adding unnecessary ‘noise’ to the chart which then takes longer for the reader to process the information. It’s also a green bar, placed next to a red bar, which would make both categories look brown to someone with colour blindness.

So how can we redo this chart? I would suggest small multiple slope charts. Here is what I came up with.

So what’s a slope chart (or slope graph)? It’s a chart that shows development over two time points. We encode the data by looking at the position of the two points and the angle of the line that joins them.

Slope graphs can have multiple categories on the one chart, like the AFL and rugby ladder examples I constantly bombard you with (see below). This example is looking at ranking over two time points.

I know what you are thinking… why have I done 5 individual charts for this course evaluation when I could just add them all on the one? Well there are a few reasons.

Firstly, to make the charts easier to read, understand and digest, I would suggest adding the values directly next to each point. In some instances, you may be able to embed the category into the chart too, next to the value (Check out this NPR example on How Each State Generates Electric Power).

In our case, each of the categories/ questions are quite long. This would make them hard to squish at the side of the post data point. Also, there are four post categories that have a value of 100%. It would make it nearly impossible adding four long questions next to the one data point, not to mention, very messy.

Having them as five small multiples, makes it easier for your audience to walk through and understand each question individually.

What else have I done to make this chart simple and elegant? I’ve used the one colour for each of the five charts to maintain a level of consistency and matched it with a grey grid. As mentioned above, I’ve added the values next to each data point, and no other numbers have been included. It’s to scale, so I don’t mislead my audience and I have a descriptive heading that introduces the data below.

Below is another example I produced for a client recently, comparing two different categories (liberal fluid strategy vs restricted fluid strategy) at two times points (pre and post surgery).

I have created both of these in InDesign, however I can confidently say you could produce a slope chart in nearly any program. In PowerPoint, draw two 10cm lines and scale your data that way (e.g 65% would be 6.5cm up the 10cm line). Here is an excel tutorial from Peltier Tech Blog.

Slope charts are an easy, simple and elegant way of displaying changes over two time points. Scrap the old-fashioned bar chart and move to something much more contemporary and effective.

In a recent presentation I gave on creating research impact using infographics and data visualisations, I displayed this terrible bar chart (found at https://viz.wtf/ – very funny site) as an example of what not to do.

As you can see it’s far too cluttered and busy and your eyes don’t know where to look. It’s hard to draw meaning from something this messy and your audience will probably give up before your message is properly communicated. We would be in serious trouble if this was to be printed in black and white. So, I suggested to the group that as we are comparing two groups, each with their own sub categories, we might think about turning this into a back-to-back bar chart. I left it at that.

After the presentation I thought I should put my money where my mouth is and actually create a back-to-back bar chart from this data to show how it should be done.

This is what I came up with...

Let me walk you through what I have done.

Before I begin, let me add that I created this in InDesign. I have included a tutorial below on how to create a back-to-back bar chart in Excel from Stephanie Evergreen.

Step 1

So first off, I organised the ‘hospital inpatient’ group from largest to smallest. This was no mean feat trying to match the eccentrically coloured columns with the tiny coloured legend at the bottom. You can see I have scrubbed out the Y axis on the left as we already have the values in the rows, we don’t need them twice. Bar graphs always look better and are easier to comprehend when the data are ordered. It’s less work for our brains to do.

I had to use a condensed font to squeeze in the 11 categories and even used some abbreviations. I then filled the data in the first bar graph. I used one colour only and put the exact values at the end of the column so there is no ambiguity.

Step 2

Step 2 was to make the next bar graph (‘residential’) on the right, making sure I had matched up the categories correctly. It was unlikely that the data would be largest to smallest in this category, but that was only necessary in the ‘hospital inpatient’ group to guide our initial design.

Again, I put the exact values into the rows so we wouldn’t need an axis or grid lines. You can now clearly see the patterns in the data and it’s a lot easier to follow and draw comparisons between the two groups.

Step 3

One thing in particular that I learnt in Stephanie Evergreen’s course was to display a heading that draws clear conclusions from your data. Tell your audience what you want them to see in the data. As I don’t know by whom or why this visualisation was created or what message they wanted to tell, I’m going to pretend and pull my own message from the data.

I have colour coded the two groups ‘hospital inpatients’ and ‘residential’ to relate to their respective bar charts so there is no need to put a key in.

If I only wanted to focus your attention, just say, on the lack of beds assigned for mental health treatment in general hospitals, I can grey out all the other data and draw attention to the ‘general hospitals’ row. This is especially effective if you are using it in a presentation. All your other data is there for comparison, but you are telling your audience exactly where you want them to focus.

So there you have it. A few easy steps to follow and we have turned something horribly busy into an easy to comprehend visualisation.

To learn how to make a back-to-back bar chart in excel, here is a tutorial from Stephanie Evergreen: https://stephanieevergreen.com/making-back-to-back-graphs-in-excel/

If you are interested to learn more about data visualisation, please take a look at my one day short course Design and Data Visualisation for Knowledge Translation. This course is designed to teach individuals ways to translate and present their work into creative and compelling visualisations. No design experience necessary.