Gradient Fill and Deception with Charts and Graphs
Below you will see a column chart that appeared in the weekend’s print edition of the Baltimore Sun. It’s no secret that they used a gradient fill on the columns to give it the fading appearance. I’m not a big fan of the gradient fill on the 2009 columns, but this could work for the previous year’s numbers (2008) if the intent was to minimize the prior year. I doubt that was the case as I’m sure they were trying to make the chart "pretty" or different than the default setup.
Below you will see a replica that I made using Excel and the fill effects formatting option. It looks alright, but something still isn’t right. What is the problem with this chart?
The problem is the y-axis and the scale that was used. I don’t think this is a straight out misrepresentation in order to mislead, but it could be. That’s the risk you face when manipulating the axis. Yes, the columns take up a lot of space when the axis starts at zero, but that’s the correct method here. To help illustrate my point, check out the exact same chart (below) with the y-axis starting at zero.
This version using the correct axis setting accurately shows that October, year-over-year, is not three times as much, but only about 1.5 times greater. Also, look at the trend of the first replica chart. The upward trend definitely has a greater slope compared to the replica with the correct axis. To help prove this visually, check out the side-by-side comparison below using a trendline in the chart. The slope of the chart on the left is much greater than the one on the right. If you were presenting this data in something like PowerPoint or SlideShare, and quickly went to the next slide, the audience might not catch the axis starting at 5 and the steep trendline would be the point taken from the data.
Furthermore, forget the gradient fill and go with something like the chart below if you want to highlight the current year.
November 10th, 2009 at 7:47 am
well, bar charts ought to start at the 0 axis. bars convey the idea of scope, amount, difference with 0.
if the minimum value of the whole series is much greater than 0, why not use a dot plot? (i.e. plot a dot at what would be the tip of the bar, instead of the whole bar) -j
November 10th, 2009 at 8:29 am
There’s a charting tutorial on the Microsoft web site that is an absolute disaster.
To make the column chart pretty, they recommend using gradient fills for both the bars and the background. Problem is, the gradients sonflict with each other, making it hard to judge the size of the bars.
To compound this sin, they recommend starting the value axis of the column chart above zero. Less wasted space or some such. If you ask me, that whole tutorial was all wasted space.
November 10th, 2009 at 8:46 am
I could never understand the fascination with gradients in charts, they add nothing but confusion to a perfectly alright chart. Just today I saw a gradient chart on Scoble’s blog .. http://scobleizer.com/2009/11/02/louis-grays-five-stages-of-early-adopterism-chart/
November 10th, 2009 at 8:54 am
Tony,
awesome job breaking down this chart. People can learn from this exercise. So many people do not know how to chart correctly and end up misleading people by corrupting the data. I wonder how many misguided executive decisions are made based on the poor use of data visualization?
@dmgerbino
November 10th, 2009 at 11:38 am
Here’s my review of the tutorial I cited above:
Misguided “Professional” Charting Tutorial
November 16th, 2009 at 4:33 am
[...] Why gradient fills can be bad for your charts? [...]
November 26th, 2009 at 12:02 pm
on http://www.hichert.com/de/consulting/managementberichte/78-diagrammstrip (http://www.hichert.com/downloads/newsletter/Chart_Strip_a.pdf)
you can download a chart-strip dealing with those issues