r - Get tick break positions in ggplot

Question

I have created a function for creating a barchart using ggplot.

In my figure I want to overlay the plot with white horizontal bars at the position of the tick marks like in the plot below

p <- ggplot(iris, aes(x = Species, y = Sepal.Width)) + 
geom_bar(stat = 'identity')
# By inspection I found the y-tick postions to be c(50,100,150)
p + geom_hline(aes(yintercept = seq(50,150,50)), colour = 'white')

However, I would like to be able to change the data, so I can't use static positions for the lines like in the example. For example I might change Sepal.With to Sepal.Height in the example above.

Can you tell me how to:

1. get the tick positions from my ggplot; or
2. get the function that ggplot uses for tick positions so that I can use this to position my lines.

so I can do something like

tickpositions <- ggplot_tickpostion_fun(iris$Sepal.Width)
p + scale_y_continuous(breaks = tickpositions) +
geom_hline(aes(yintercept = tickpositions), colour = 'white')

Answer 1

A possible solution for (1) is to use ggplot_build to grab the content of the plot object. ggplot_build results in "[...] a panel object, which contain all information about [...] breaks".

ggplot_build(p)$layout$panel_ranges[[1]]$y.major_source
# [1]   0  50 100 150

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See edit for pre-ggplot2 2.2.0 alternative.