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I was just wondering if you can have different units on the same y-axis? For example, if I'm graphing blood flow, oxygen level and skin temperature against time in one graph. I can put temperature on the secondary y-axis, but I would still have two variables with different units on the primary y-axis. Will this be alright? Or do I need to draw a separate graph for the other variable.

Julia Yin
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You can use separate y axes, with different units. For 2 variables, you can use axes on the left and right side of the graph. For more than 2 variables, put 2 or more y-axes next to each other with different labels.

Just make sure the reader knows which axis to use for each data point. This will be easier if you use data curves rather than data points.

And, you should have a reason for using one graph instead of several. For example, in your case there may be a correlation/causation between blood flow and oxygen level. A combined graph makes it easier to show that correlation.

Here's an example (from https://help.plot.ly/excel/graph-with-multiple-axes/ ): enter image description here

hdhondt
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  • Thank you so much! So that means one y-axis can only have one units, and if I want to put different units I need to have different y-axis for each unit? – Julia Yin Apr 09 '18 at 10:30
  • Yes, use separate y-axes for different units. Depending on the application used, you may be able to have multiple axes side by side. For more examples, search google images for "graph with multiple y axes" – hdhondt Apr 10 '18 at 02:17