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The case against dual axis charts (and what to use instead) (2018)
- cs702 2y agoI'm 100% in agreement. To anyone here who thinks plots with two different scales in the same direction sometimes are appropriate: Please read this. --- EDIT: Changed "dual axis charts" to "plots with two different scales in the same direction," which more accurately describes the OP's topic.
- Scene_Cast2 2y agoWhat about Bode plots?
- cs702 2y agoI have nothing against them. Please note, I edited my comment to change "dual axis charts" (common spreadsheet terminology) to "plots with two different scales in the same direction," which more accurately describes the plots with which the OP -- and I -- disagree.
- isidor3 2y agoPlotting the average or top percentile latency of an API on the left axis and the number of calls to that API on the right is pretty much standard practice where I work. I would argue it makes things more clear. You get to see exactly how the latency changed as the traffic does, or where more noise is visible because the traffic was low. Because both scales are using completely different units it's more difficult to confuse the two.
- Leftium 2y agoFound this while trying to create an observable plot with multiple scales[1][2]. I'd argue multiple scales are OK if the multiple axes have different units that can't be easily compared/confused and are used for greater information density (instead of relative comparison purposes). For example: I'd like to plot weather stats like hourly temperature, precipitation, and AQI throughout the day, so several different days can be compared with each other. (And fit all this information on a mobile screen.) [1]: https://github.com/observablehq/plot/issues/147 https://github.com/observablehq/plot/issues/147 [2]: https://github.com/observablehq/plot/discussions/626 https://github.com/observablehq/plot/discussions/626
- bradford 2y agoI was coming here to say something similiar. The article only shows examples of dual axis charts where line series are used for both axes. This will clearly cause confusion (especially when tooltips are not available). I've generally found that when displaying a percentage, it is helpful to show the individual counts for numerator/denominator. I believe that showing percentage as a line series on one axis, and raw counts, represented as a column on the other axis, can be a helpful visual.
- dendrite9 2y agoHave you looked at how yr.no plots their forecasts on the phone. I find the graph view to be very helpful in the winter when planning ski days or days to skip. They must have updated the charts recently since they are different from what I remember this winter but it looks similar. An example: https://www.yr.no/en/forecast/graph/1-305409/Norway/Troms/Troms%C3%B8/Troms%C3%B8 https://www.yr.no/en/forecast/graph/1-305409/Norway/Troms/Tr...
- o0-0o 2y agoGreat article. I used to be into charting a lot and ran a charting product at a famous firm. Would love to see the thoughts of the author on other charts like radar and treemap. :) Great read.
- einpoklum 2y agoWe could really use your help in LibreOffice! I mean, if you are still into coding. There is quite a bunch of work to do on charting: Desired chart enhancements in general https://bugs.documentfoundation.org/showdependencytree.cgi?id=108538&hide_resolved=1 https://bugs.documentfoundation.org/showdependencytree.cgi?i... Desired additional chart types https://bugs.documentfoundation.org/showdependencytree.cgi?id=159181&hide_resolved=1 https://bugs.documentfoundation.org/showdependencytree.cgi?i...
- goldemerald 2y agoSolution 4 is so hilariously bad I am shocked it was suggested. Building a 2d landscape where the time dimension seems to take a random walk made laugh a lot. Ignoring the standard convention of "independent variable on x-axis" and instead embedding it as datapoints is a particularly clever way to obfuscate the data and confuse the reader.
- deleted 2y ago[deleted]
- sixthlime 2y agoI thought so at first too, but if you look at the link they included [1] it seems like it can actually be quite clear for some datasets [1] https://archive.nytimes.com/www.nytimes.com/imagepages/2010/05/02/business/02metrics.html https://archive.nytimes.com/www.nytimes.com/imagepages/2010/...
- msm_ 2y agoI don't agree. It's a great way to visualise data when you want to focus on a trend. It makes it very obvious which "direction" is the data heading. But of course it is not very often used, is not a great fit for every use case (in particular, bad for the data in OP) and may be confusing when seen first time.
- einpoklum 2y agoThe first couple of arguments are weak: 1. It's possible to mislead by playing with a single series' scale, you don't need two series to lie-with-statistics... 2. The argument that people will think the data are identical despite the different scale? Don't buy it.
- parpfish 2y agoAgreed. dual scale plots are a great visualization to emphasize correlation between time series. I think of it as depicting an intermediate step in computing a Pearson R when the data have been z-scored but before you’ve collapsed across data points
- hex4def6 2y agoAs an engineer with an oscilloscope, not being able to plot two probes against each other on the same chart would be severely limiting. For instance, imagine a 10x attenuator / amplifier. Maybe the input has a DC offset. Being able to plot the two against each other to look for (e.g) distortion, is invaluable. This is committing the two cardinal sins (judging from some comments here) of not starting at zero and different scales, and yet it's not misleading at all. I can believe dual axis charts allow misleading results, but that doesn't mean they don't have completely legitimate uses.
- deleted 2y ago[deleted]
- tofof 2y agoAnd the possibility of fiddling with scale to mislead still exists with side-by-side charts, their #1 alternative. In fact, they use the same misleading scale start and stop points as they criticize in the dual-axis version, so that the "one went up 80%, the other went up only 40%, but it looks like they went up equally" still applies to their replacement.
- ajuc 2y agoIn case of German GDP vs Global GDP I'd argue the correct thing to do is to draw a graph of a new variable "German GDP as a percentage of Global GDP" and a separate graph of Global GDP.
- pasc1878 2y agoI would be the one in the sample who did not find the charts confusing. The two separate graphs are much more difficult to compare - you can't see which elements compare to the same year so lose a lot of information. The information in the chart is if there is a change in one time series is there a change in the other. - that is probably all you can infer as without error bars you can't see if the differences are material. (ie I know they are different scales so when they cross they obviously aren't the same.) If so there might be a correlation which might be worth looking into remembering correlation does not equal causation (so the example in the link are just laughable) The prioritisation just shows nothing. The scatterplot shows nothing The indexed chart does make sense and in this case I would agree would be better.
- ericpauley 2y agoThe connected scatter plot is so cursed... talk about hard to interpret! The only conclusion that's hard to argue with here is zero-indexing plots, but that's not exactly a new finding.
- pasc1878 2y agoAlso the scaling - in this case the original had reasonable scaling but it can be manipulated. The changes could be small enough to be random fluctuations on one series and so no real match. However the graph does show that a slightly deeper look would be worthwhile - even if it is a very quick one to see that the data is manipulated e.g. climate deniers graphs of temperature all starting on the same year. If you change the starting year you got rather different results.
- parpfish 2y agoI could see that connected scatter MAYBE working if it were an animation or interactive plot. Maybe. But on its own, it’s horrid
- slow_typist 2y agoThe problem with indexed charts is that the base year is arbitrarily set and can change the whole picture a lot.
- deleted 2y ago[deleted]
- jrd259 2y agoI'd argue that the zero value should always be shown. Otherwise you get different impressions of the rate depending how you scale and subset the Y axis.
- PheonixPharts 2y agoThis is not a good practice at all. Do you think atmospheric CO2 charts should show 0? How about daily temperature reading for human body temperature? Should daily stock tickers all start at 0? Why is 0 magical? Adding 0 to the vast majority of plots shows that data at an unnatural scale that can obscure genuinely important information. Human body temperature readings on a scale from 0 to 107F would make all the important information hard to see. A much better rule is that charts should have reasonable bounds based on knowledge of the system. For human temperature in F anything less that 95 and greater than 107 basically mean you're dead. For processes in nature good points are some delta - the lowest record to delta + highest recorded. For things like daily stock prices, a few standard deviations each way from historic volatility works. The dogma that charts should all start at 0 is complete nonsense and tries to side step reasoning about you data. Yes scales can be used to misrepresent data, but forcing 0 to the axis does not solve this.
- yau8edq12i 2y agoFahrenheit is not an absolute scale, so there is nothing special about 0F, you're right about that. As for your other two examples (atmospheric CO2 and stock tickers)... Yes, the scale should start at 0. Why shouldn't they?
- rossdavidh 2y agoSo, it's great that they try to actually get data on what kinds of charts convey what information. However, you need to know who your audience is. I, for example, found all of their suggested alternatives to be harder to interpret than the dual axis chart. If you're trying to see whether or not the ups and downs of two different variables are similar, suggesting a connection between the two, none of the suggested alternatives do as good a job (although two charts could, if instead of having them side by side you had them one above the other, with the same x-axis scale, but that is really just a stealth dual axis chart). Most of these "don't use this kind of chart" seems to be trying to make it impossible to confuse or mislead your audience, and that is just not plausible. You do, and probably usually should, have some point in mind when you are showing someone else a chart, and the format needs to make it easy to see that. Almost any chart, even pie charts, have some particular use case where they are the best chart for that purpose. No chart is going to always be the best way to present data. Like choosing what kind of language to use in explaining something, you need to know something about who your audience is, and what they are accustomed to.
- petsfed 2y agoWasn't there an article the other day about a concept that's similar to incompleteness theorem? That any ambiguity-free language is incapable of completely describing sufficiently complex situations? Am I just imagining that? [0] I feel like making a tool harder to use, just to prevent bad actors, only punishes good actors, while the bad actors find some other way to act badly. Like, I don't want to participate in your arms race against disinformation purveyors, i just want to illustrate that it tends to rain on days that are cloudy and have high humidity. 0. Sort of. I recently encountered "Colorless green ideas sleep furiously" (https://en.wikipedia.org/wiki/Colorless_green_ideas_sleep_furiously https://en.wikipedia.org/wiki/Colorless_green_ideas_sleep_fu...), although where I can't recall, and sort of inferred the rest.
- rossdavidh 2y agoRegardless of whether or not there was such an article, I think the principle is the same, and it's a good analogy.
- joshe 2y agoContext is important, this is targeted at journalists. They are usually trying to make a point to casual readers. For readers with more interest or who are numerate in their day jobs (engineers, finance, or economists), dual axis charts can often be a great choice. This is better graph style advice from the Economist, which includes good dual axis examples and one bad one and how to correct it. https://medium.economist.com/mistakes-weve-drawn-a-few-8cdd8a42d368 https://medium.economist.com/mistakes-weve-drawn-a-few-8cdd8... Since we are engineers or founders trying to deal with very complex systems, adding detail and clarity like the Economist or Edward Tufte does is the better way to go.
- lisacmuth 2y agoAuthor here. Thanks for setting the context: Datawrapper – the data vis tool I write articles like this for – is indeed for people who want to make a point with their charts and maps, often to a broad audience. I agree that people who have learned to read dual axis charts can benefit greatly from them (the same is true for rainbow color maps). Financial Times journalist John Burn Murdoch changed my mind on dual axes charts – even for casual readers! – a bit over the last six years, too. Here's a dual axis chart he created for the FT: https://x.com/AlexSelbyB/status/1529039107732774913 https://x.com/AlexSelbyB/status/1529039107732774913 The next article I write on dual axis charts will probably be a "What to consider when you do use them" one.
- erehweb 2y agoI get it, and sympathize, but at many companies the decision maker is someone who wants to see dual axis charts. If Datawrapper can't do that, then that would be a point against using it widely.
- mst 2y agoIt would be a point against using it in companies where the decision maker thinks they know better than the people making the charts, certainly (though if you've managed to already adopt the tech, you can often present decision makers with an alternative (indexed charts for most cases, I suspect) and they'll turn out to be more flexible than you'd originally guessed). I think "trying not to add things they consider footguns so the people making the charts are happy and so more likely to recommend them at their next job" is a trade-off that may or may not turn out to be more profitable in the long run.
- patrick451 2y agoWhat a patronizing company. Your customers keep asking for a feature that is widely supported and you refuse to add it because it violates your sensibilities. Instead, you write this diatribe lecturing us that the way we want to display data is wrong. Just reading the opening paragraph, whatever interest I may have had in your plotting capabilities evaporated.
- jdeaton 2y agoI once worked with someone who was doing performance benchmarking of two systems, and made a duel axis chart with the lines right on top of eachother when in fact one system was like 5x faster than the other. it drove me nuts because I didn't even realize the dual axis at first and thought that they literally had identical performance
- jszymborski 2y agoYah, the moment you see/make a dual-axis line plot, you know you're comparing relative change. The whole point is to effectively normalize for absolute value. So yah, anyone arguing primarily on the basis of absolute value on these plots is likely pulling a fast one.
- aj7 2y agoNo. You’re missing two arrows, each near its graph and pointing to the corresponding y-axis. This has more impact than color matching.
- tqi 2y ago1. I'm not sure why having two charts side by side helps? 2. Indexed charts are also not a panacea - depending on what point on the x axis you choose as your starting point, it is easy to make it seem like one series is rapidly outpacing the other (ie choosing to start at the peak). Ultimately I think charts are best thought of as a way to communicate a conclusion, not be the primary source for drawing a conclusion. Figure out what point you are trying to convey and choose the chart that communicates that the best.
- Too 2y agoYeah. If you are going to be tampering with the zero offset, the side by side charts have exactly the same issue of misleading.
- Tarq0n 2y agoGrowth is actually one of the few things where it's permissible to remove 0 from a scale. For instance with asset prices, the dollar value doesn't matter, only the magnitude of the change.
- thedudeabides5 2y agoEh, how about look at the axis units when someone is pitching you something.
- treflop 2y agoI find plotting two correlated but unrelated data series (like temperature and humidity) can be fine. But the article chooses the worst possible dual axis charts where both data series not only measure the same thing (GDP) but share the same units (dollars). What you actually have is a multiple data series chart with actually one axis but you made two to be confusing.
- ghssds 2y agoAn article about how to do charts, which itself uses charts with labels so small they are unreadable while also disabling zooming, lack credibility.
- gcanyon 2y agoSomething that occurs to me: there's [not much|nothing] special about "dual" axis charts, meaning: in general the graph matches color with the axis, but there's nothing "right" vs. "left" about the graphs. Therefore, there's nothing special about "dual" axis charts being right- and left-axis, and you could just as easily put both axes on the left (at a little loss in clarity). And finally, that means you could just as easily have three, four, or more axes. Not really -- obviously a chart with six axes on the left and six different color-coordinated graphs would be absurd, but that's my point: there's nothing that makes a two-axis graph less absurd other than scale. All of which to say: if there's no real correlation that you're trying to illustrate between the data series, then separate charts are the way to go. If there is a correlation, there's (probably) a better way to illustrate it.