Comment by PheonixPharts | Hacker News Reader

Comment by PheonixPharts | Hacker News Reader

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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.

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.

What's a good discussion of the pros and cons of pie charts?

Edward Tufte is a great source for learning which types of charts and visual techniques do their job best. I enjoyed his book "The Visual Display of Quantitative Information."

Here's a discussion thread about pie charts on his website (Tufte chimes in a few times): https://www.edwardtufte.com/bboard/q-and-a-fetch-msg?msg_id=...

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?

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.

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.

> 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?

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.

If 0 is not useful reference, then the chart should have the X axis on top, to avoid the intuitive tendency to interpret the graph as a heigh chart.

That’s too weird for most audiences. Removing the X axis altogether seems more appropriate (while keeping the labels). Then the plot area is still “bottomless” in a sense, but the labels are where people expect to see them.

Having the axis on top implies that the values are negative. Like an ocean depth chart.

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