What is Y-axis in this chart?

[21-Hidetaka-Ko]

Hi Ross,

I enjoyed your blog post on the iPhone step data. Good stuff!

By the way, I have a question about your blog. What is Y-axis in this chart?

[rosskipp]

Hey!

Glad you enjoyed the blog post! 😄

This is a density plot - the ggplot docs say Gaussian kernel density estimation for distribution of x parameter. So the Y values are the densities or likelihoods of the x parameter, in this case steps, at a given value. An alternative to this could have been two histograms to show the distribution differences, but I thought the lines would be easier to differentiate.

Here's some more info if you want to read more:
https://en.wikipedia.org/wiki/Probability_density_function
https://en.wikipedia.org/wiki/Kernel_density_estimation