[HN Gopher] The case against dual axis charts (and what to use i...
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The case against dual axis charts (and what to use instead) (2018)
Author : Leftium
Score : 77 points
Date : 2024-05-17 16:30 UTC (6 hours ago)
(HTM) web link (blog.datawrapper.de)
(TXT) w3m dump (blog.datawrapper.de)
| cs702 wrote:
| I'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 wrote:
| What about Bode plots?
| cs702 wrote:
| I 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 wrote:
| Plotting 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 wrote:
| Found 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
|
| [2]: https://github.com/observablehq/plot/discussions/626
| bradford wrote:
| I 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.
| o0-0o wrote:
| Great 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 wrote:
| We 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?i...
|
| Desired additional chart types
| https://bugs.documentfoundation.org/showdependencytree.cgi?i...
| goldemerald wrote:
| Solution 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.
| sixthlime wrote:
| I 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/...
| msm_ wrote:
| I 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 wrote:
| The 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 wrote:
| Agreed. 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 wrote:
| As 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.
| tofof wrote:
| And 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 wrote:
| In 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 wrote:
| I 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 wrote:
| The 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 wrote:
| Also 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 wrote:
| I 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 wrote:
| The problem with indexed charts is that the base year is
| arbitrarily set and can change the whole picture a lot.
| jrd259 wrote:
| I'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 wrote:
| This 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 wrote:
| Fahrenheit 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?
| parpfish wrote:
| Because starting at zero can cause scaling issues that mask
| meaningful trends and variation. That can also be abused to
| mislead, but a simple rule like "always include zero" ain't
| the solution to that.
| jrd259 wrote:
| All fair points about zero. Sorry, I acknowledge now I
| was overly influenced my metrics dashboards I use for
| alerting. I've seen people panic at a seeming steep rise
| in error rate or increase in latency because the chart
| was not showing the full range (0 to 1 for rates, or 0 to
| 2x SLA for latency). I was only thinking of operational
| alerting dashboards.
| PheonixPharts wrote:
| > Fahrenheit is not an absolute scale
|
| So if someone showed body temperature measured in _Kelvin_
| you would argue that it _should start at 0_? That seems
| even more ridiculous.
|
| > Why shouldn't they?
|
| Because for the vast majority of stock it would appear to
| be a straight line every single day? Can you find me a
| example of a stock trading app for a company who's price is
| > $100/share that shows intraday price activity on a zero
| scale?
|
| Likewise most co2 charts start around 300ppm since that has
| been roughly where the lower bound of atmospheric co2
| levels have been for all of human history.
|
| The last time co2 was 0 on the planet earth it was just a
| molten rock so what's the _meaning_ of showing this value?
| It 's not even theoretically possible that co2 could be
| that low baring alien life sucking the atmosphere off the
| planet.
|
| Can you clarify why the scale _should_ start at 0 for these
| things? How is that anywhere close to an honest
| representation?
| hex4def6 wrote:
| In that case, we should report body temperature in Kelvin.
| However, now the dead-alive range (95degF - 107degF)
| becomes 308K to 315K.
|
| Starting at zero, that range (17K) is now only 5% of the
| graph, assuming we start at zero. Or in other words, if
| your chart is 10cm tall, the entirety of the useful range
| is compressed into a space that is 5mm tall.
| vharuck wrote:
| Yes. Charts are communication devices. Any "rules" for charts
| should be seen like similar "rules" for essays or emails:
| good advice that almost always gives a satisfactory result
| when followed. Reliable paths for infrequent authors.
|
| But what matters most in charts is the same thing that
| matters most with writing: pick one major point and stick to
| it (if you're really good or can't avoid it, maybe a couple
| points). This also explains why a lot of dual-axis charts
| don't work: the author explains two sets of data that aren't
| even measured on the same scale and then leaves the reader to
| connect them _and_ understand the meaning of that connection.
| You can 't be sure the reader will end up at the point you
| wanted to make.
|
| That's not to say a dual-axis chart is always the wrong
| choice. Just that, if you start making one, stop and ask if
| there isn't a better way to show the data. Same with pie
| charts.
| rossdavidh wrote:
| So, 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 wrote:
| Wasn'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_fu...), although where I can't recall, and sort of
| inferred the rest.
| joshe wrote:
| Context 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-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 wrote:
| Author 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
|
| The next article I write on dual axis charts will probably be a
| "What to consider when you do use them" one.
| joshe wrote:
| What a great update, thanks for posting!
| erehweb wrote:
| I 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.
| patrick451 wrote:
| What 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 wrote:
| I 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
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