Y-axis of a graph with a common unit but different meanings


I have a graph where the x-axis is time and y-axis is the density of two or more different organisms. So the graph shows how the densities change with time. Suppose the species are tunas and whales, and the plot shows the data for the both species together. Should the label for the y-axis Density or Densities? I have seen both ways...


Posted 2014-08-04T01:48:14.743

Reputation: 835



Density. The axis is only a measurement scale. At any given time, each data set has only one value.


Posted 2014-08-04T01:48:14.743

Reputation: 29 679


You should label it density unless you're doing something really peculiar.

At any given point of time, an animal (or group of animals, if this is an aggregation or population density) possesses a single density. They probably possess multiple units of density measurement - e.g. Wikipedia lists the average density of a human as 1062 kg/m3 - but only a single density*.

Your Y axis counts units of density measurement, not numbers of densities; it quantifies the quality rather than asking how many different instances of that quality exist. Unless you're using density as the metric for determine number of populations of animals (e.g. a density of 10 animals/km2 and another location having a density of 15 animals/km2 demarcates them as different populations). But that's a really crazy way to count population groups.

* - Yes, I realize the density will be different at different places in the body, but at that level we're talking about tissue densities, not human density. Even a heterogeneous body has only a single density when considered as a whole. I also realize this is most likely about population density, but it's a lot cooler to me to think of it in terms of physical body density, and that doesn't change the answer, or even the above asterisk (with appropriate terminological substitutions of course).

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Posted 2014-08-04T01:48:14.743

Reputation: 7 480