# 2 Y Axes in Algebra of Graphics

**URL:** <https://discourse.julialang.org/t/2-y-axes-in-algebra-of-graphics/68968>\
**Category:** Visualization\
**Tags:** algebraofgraphics\
**Created:** [September 29, 2021, 10:26pm UTC](https://discourse.julialang.org/t/2-y-axes-in-algebra-of-graphics/68968 "2021-09-29T22:26:19Z")\
**Posts on this page:** 1\
**Showing post:** 4

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**Author:** ![Eben60](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/eben60/32/13475_2.png) [@Eben60](https://discourse.julialang.org/u/Eben60)\
**Post date:** [December 6, 2025, 11:39am UTC](https://discourse.julialang.org/t/2-y-axes-in-algebra-of-graphics/68968/4 "2025-12-06T11:39:02Z")

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> [@dlakelan](#):
>
> For example suppose the two things you want to plot are the angle of a linkage rod that varies over about 10 degrees, and the force applied to it, both as time series… Instead of plotting say degrees and kilonewtons you take the data and convert to degrees/max angle, and force as a fraction of peak force for a cycle… Now you have values that are all in the range of about ±1 and can plot on a single dimensionless axis

Yes, using reduced values is a great way to unlink onself from the details of the reality 😉

Sometimes it is useful, and a theoretical physisist would happily use `eV` units for everything (`1 kg = 5.61e35 eV` , `1 s = 1.52e15 1/eV` ). In practice however, there are practical reasons for using practical (and even imperially-unpractical) units. I have a good feeling for what a `kN` is (disclosure: it is about 1.5 of my weight), and what a `°` is (I can draw an angle by hand to a few degrees exact), and data in `°` and `kN` is informative for me on multiple levels. Having values plotted as reduced to some random “`100%`” takes some crucial information out of the visual representation. Sure, being old school I can multiply `30%` by `1.47kN` in my head, but do I need that additional mental load? And how should we plot after you shiift the gear of your machine?

Should we take a more realistic example? A common case - you have some process of interest, and there is some changing environmental parameter. E.g. you observe the course of a chemical reaction, measuring some concentration over time, whereas the temperature changes, too. To make sense of what is happening, you need **both** data, preferably **on one single plot** , and definitely **in units** your technician can understand.

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