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Comment by ppqqrr | Hacker News Reader

A while ago i had a heated discussion on HN with someone who claimed that any graph where 0 is not the minimum value of all the axes is misleading.

We were talking about a graph that shows global temperature rise due to climate change. They claimed the graph was misleading because the Y axis (temperature) didn't start from 0 (fahrenheit? celsius? fucking kelvin?).

This person also quipped, "maybe if you can't see if with 0 at the bottom, it's not such a significant change?". That put a dent in my faith in humanity for a while. I'm just glad to see us operating at a higher level. I guess 2016-2020 was a different time.

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You should hand him a chart with "years" on the x axis with it starting from 0AD. :P

Why some random starting point like when a religion started counting? Ofcourse you have to start from the beginning of the universe. THAT will put things in perspective!

The only sensible start for any time axis is clearly Modified Julian Day 0 which puts the x=0 of any truly god fearing years axis in November 1858, as it should!

Alternatively Fermi Mission Elapsed Time is also an acceptably cool zero point, which puts the zero in January 2001. The zero of the unix time is tolerable only in truly desperate circumstances.

>fucking kelvin?

I mean yes, if you want the ratios of different temperatures to be meaningful, then that's where you'd need to set the zero point. You could argue that a graph that makes 25C look "25% hotter" than 20C is misleading in this sense. (Not that this justifies global warming denialism.)

Significant changes are not necessarily visible on a scale from 0 K to 400 K. I mean, if you show up at the hospital with a temperature of 315 K instead of your base line around 310 K, that’s fucking significant even though you would not see anything on a scale that starts at 0 K.

> You could argue that a graph that makes 25C look "25% hotter" than 20C is misleading in this sense.

That’s meaningless in any sense. The origin of the Celsius scale is arbitrary, “25% hotter” has no meaning whatsoever.

>The origin of the Celsius scale is arbitrary,

True – but the origin of the Kelvin scale is not.

>“25% hotter” has no meaning whatsoever.

It absolutely does have a physical meaning. It means that the system has 25% more energy at the microscopic level. (Or, you know, substitute in a more precise physical definition of temperature – it will be some kind of measure of energy, even if it's not exactly that.)

It's not necessarily wrong to suppress the zero in a graph of temperature changes, but by doing so you are making bars in the graph proportionally larger or smaller relative to other bars by an arbitrary amount. That could potentially be misleading, depending on what point you are making using the graph.

> It absolutely does have a physical meaning.

No, not really. Heat is a poorly defined concept to which we are saddled for historical reasons. For example:

> It means that the system has 25% more energy at the microscopic level.

It does not. This definition only works in a frame of reference at rest compared to the thing you are observing. Imagine a piece of matter that is travelling at a velocity v in your implicit frame of reference. Its temperature does not depend on v, even though it’s kinetic energy does. We are back to the choice of scale.

And then there are negative absolute temperatures, which do not make any sense at all if heat is kinetic energy.

The actual definition of thermodynamic temperature is the inverse of the derivative of the energy with respect to the entropy. This is highly non-intuitive and we cannot extrapolate our intuitive concept of heat too much.

> It's not necessarily wrong to suppress the zero in a graph of temperature changes, but by doing so you are making bars in the graph proportionally larger or smaller relative to other bars by an arbitrary amount.

Right. This is the point that was made in the story and I entirely agree with that. A bar chart communicates a surface area. Changing the scale artificially changes the surface area and is misleading. The logical conclusion is that bar graph make no sense for temperatures, or to show the relative change of a variable.

Personally I would go further and say that bar graphs are inappropriate in the vast majority of cases, but that’s just my opinion.

> That could potentially be misleading, depending on what point you are making using the graph.

Indeed.

If we're talking about global warming (as ppqqrr was), then it's surely some kind of objective physical notion of 'getting hotter' that we're interested in. The problem with global warming is not that we all feel subjectively hotter!

And I did say:

>substitute in a more precise physical definition of temperature

Easy solution: just plot the delta in °C since some fixed date. (Or for any graph, just subtract y(x_0) from every point. Tada!)

To add, that's actually a Bett chart anyway, because you aren't showing the temperature over time, you want to show the change in temperature over time.

Completely different things.

"that any graph where 0 is not the minimum value of all the axes is misleading."

I partly agree with him.

Take this example, first graph I could find:

https://religionnews.com/wp-content/uploads/2014/08/61Years-...

For me it looks on a first look like a 2/3 decline and this is misleading. Often this decling graphs give an optical picture that does not reflect real decline.

> I partly agree with him.

It is not helpful in general. Magnitude and relative changes are different things. Sometimes you need one, and sometimes you need the other.

Global average temperatures are a good example: where is the zero? Is it significant? The effects of an increase or decrease of 0.5°C are massive, so the appropriate way of presenting this is to show the temperature anomaly, not the absolute temperature. Also, this way the information is conveyed regardless of the temperature scale in use.

The religiosity graph is interesting. If you want to show a sudden change at some point, then showing the relative change is appropriate. If you want to show that people are not religious anymore using this graph, then you are dishonest. It is all about the narrative and the point you want to make.

On its face, “scales must go to zero” is not good advice, because you can always change the variable so you can make anything go to zero without changing the shape of the curve and our perception. However, when we see a graph, then we always need to understand why it goes to zero or not, what the author is trying to show, and whether they are being honest about it

"global average temperatures are a good example: where is the zero?"

I don't know.

"The average surface temperature on Earth is approximately 59 degrees Fahrenheit (15 degrees Celsius), according to NASA"

But I know if the average temperature increased 0.5 degree C and I show a graph with the scale 14 to 16 degrees over time and the headline "world average surface temperature exploding" then this is excellent clickbait and a nice graph, but it is misleading on the first look.

Classic example of someone taking away an overly simplified rule from a problem they don't fully understand. A little knowledge is a dangerous thing.

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