What are the most common types of time-based KPI charts do you normally see? There are many variations of time-based KPI charts. In this post, I shall attempt to analyse each type that I developed throughout my career. I shall describe each type, when it is appropriate to use them, and any limitations they may offer.
Type 1: Bullet Graph with Sparkline
The bullet graph is a very useful visual developed by Stephen Few, explained in detail on his website, Perceptual Edge, here in this article. It is very uniquely useful compared to other charts because it allows for the presentation of multiple data points in such a small space. In just a tiny display, we can observe the KPI measure, its target, and the performance range. My use of coloured value of red, amber, and green for the KPI performance ranges may not be fully in compliance with bullet graph’s accepted specification. However, since KPI performance indicators are usually represented by the familiar “traffic light” colours, I have made the exception to use those colours in this post. It also allows easier comparisons between all the different types of charts here.
On its own, a bullet graph is appropriate to show only one value of the time dimension. In the image above, this time value is December 2010. As this post is about time-based KPI charts, in the same image on the right hand, you will see the past trend of the KPI performance in a minimal line chart called sparkline. The sparkline is the invention of Edward Tufte. It is constructed without the display of axes or gridlines, and is principally used to show the general trend of a particular measure. In this example, I have added a thin horizontal line to represent the KPI target as comparison with the actual performance.
When to use …
The time-based bullet graph chart is best used in a dashboard visualization, especially when periodical reporting, such as month by month, is required. The bullet graph will represent the current reporting, such as month, while the sparkline will show the general past trend of a pre-determined time range. Naturally, with these kinds of reports, you would not have just one KPI in the dashboard; you would have a few others as well. Refer to the small example below representing a part of a dashboard with several KPI measures.
Here you can see the beauty of the bullet graph. Each of them has different range on the x-axis, different targets, and different KPI performance ranges; all beautifully represented in such a small space. This is more complicated if done using line chart for example. We would not have the liberty of displaying different x-axis scale in a small space such as this. For the sparklines, since we do not need to show axis scales and they do not need to be the same, the general trend of each sparkline shows beautifully.
As you would guess, the greatest strength of bullet graph and sparkline, specifically to this example, is also its weakness. We cannot represent the KPI performance ranges in the sparklines as efficiently possible. This is due to the small space of the sparkline would made identification and visualization difficult for human comprehension, especially on a dashboard. Check out this picture below.
Forgive the small chart display, but it is required in making this point. The KPI performance ranges on this sparkline may be visually visible to us in this post. But on a dashboard, they may look even smaller due to space limitations and thus would render the ranges virtually indistinguishable. Therefore, I generally recommend not using KPI performance ranges in sparklines. However, if a dashboard contains limited amount of information such that the sparklines can be of decent size and do not impair the aestheticness of dashboard, then by all means, KPI performance ranges may be welcomed as it enriches the general trend of the sparklines.
The other limitation has to do with variable ranges in the KPI performance indicators. I will explain this when we get to Type 3 section below.
The other types below would eliminate all the limitations mentioned above, albeit with some other disadvantages which will be explained.
Type 2: Line Chart with Fixed Target and KPI Indicators
What if you want only one chart displaying the trend of a KPI performance without the use of a combination of two charts, like the bullet graph and sparklines in Type 1 above? Then, in that case, you can utilize a single chart. Line charts would be ideal for this, since they are usually used to represent trends.You will also want the KPI traffic light indicators to be shown on the line chart as well. There are several methods of displaying this type of chart, as shown above in Charts A to C.
Chart A is the cleanest looking to me. The mark labels cleanly shows the performance rating without any possible perceived interference from the performance range separators in Chart B and Chart C. Both these charts, however, show the separations of the performance ranges without any doubt, as might be possible with Chart A. In Chart B, there might be some ambiguous perceptions as to the boundary between the KPI performance ranges. The chart legend differentiates the boundary between red (poor), amber (average), and green (good) indicators but not as clear as Chart C, which negates all doubt of the boundaries.
When to use …
These types of charts are great for display on presentation slides to KPI performance review management committee, where more details are required than dashboards could provide. In a slide, this chart can be used on one side of the slide while explanations on certain trend of the chart can be used on the other side, such as the example below.
On the other hand, if space permits, this type of chart can still be used on a dashboard. In Type 1 above, I showed an example of three KPI measures using bullet graphs and sparklines – revenue, profit, and sales. If space permits on this dashboard, or if there are additional (more detailed) dashboards for each of the three measures, then individual line charts may be used within the dashboards.
The limitation of this chart is that it requires quite a bit of space to be properly legible. As mentioned above, some dashboard designs may not permit large space for charts.
Type 3: Line Chart with Variable Target and KPI Indicators
Type 2 above deals with KPIs with fixed range in their performance targets and traffic light ranges. However, I am certain that in your career, you have encountered some types of KPIs where the targets change throughout time, either planned or otherwise. Displaying the performance trend of these types of KPI needs to take account of the difference in targets in performance ranges. Charts X to Z above shows several options on how this could done.
In Type 2 above, I can still recommend all Charts A to C to be used in some capacity or another. For Type 3 however, I can recommend only Charts Y and Z to be used, which Chart Z being the best. Chart Z uses the advantage of area charts to demarcate the variable KPI performance ranges while not interfering with the actual vs. target line charts. Chart Y offers a cleaner display without displaying the performance ranges explicitly. Chart X display of performance ranges as line charts themselves interfere too much with the actual vs. target line charts that the entire chart looks messy. Therefore, I do not recommend Chart X to be used at all.
When to use …
Like Type 2 above, this chart is best used in slides where detailed performance reviews are usually conducted, as shown below.
They can also be used in dashboards if space permits, similar to Type 2 above.
Variable ranges such as shown here require massive space to be properly viewed and differentiated. As such, if space is not permitting, displaying this chart on a dashboard may be counter-intuitive. This is more prominent in the case of sparklines, where it would be near-impossible to distinguish the performance ranges’ boundaries, as shown below.
I have shown you the different types of how trends can be displayed for KPI performance measures. I hope you find them useful. If you have any comments, or information on better displays that can be used to represent KPI trends, please share it in the comments below.
On a side note, line charts are useful for displaying trends, but not so much on volume of data. To that end, we may use bar charts to display the same data. Bar charts focus the audience’s eyes more on the data volume than the trend. In the Type 2 example above for the measure, revenue, we can display the same data in bar charts as below.
As you notice, the display works well and focus our eyes more on the data volume than the trend.
I believe that, like any chart design, it is at the discretion of the chart maker. The important thing to remember is that the design must be suited to the audience without having them to think about understanding the chart design. The audience need only to concern themselves with the message behind the chart.