Reading in R a Seasonal Polar Season Plot

Question

I'm trying to understand how to read the amount being measured in a Polar Seasonal Plot.

REPREX:

library(tidyverse)  
library(fpp2)       

ggseasonplot(a10, polar = T)

RESULT:

For example, by looking at the plot how would I know the amount (in millions) that was sold on July 2008? On the left hand side of the plot I only see two dashes for 10 and 20 million, should the distance between 10 and 20 be my scale of 10 million?

Answer 1

This does seem like poor plot design - it's more common in polar plots to label lines directly. I think the 10 and 20 are indicating distances from the center horizontal line (Oct-May), and if you imagine horizontal lines extending from the 10 and the 20, those line up as tangent to the faint gridlines. Unfortunately, the 10 is closer to the center than the 20, suggesting either the scale isn't linear or (more likely) doesn't start at 0 in the center.

I've annotated the plot here with the horizontal lines and circling the tangents.

You could use annotate or geom_text to add labels directly to the plot, and use theme() to make the grid lines a little more obvious.