r - Adding minor tick marks to the x axis in ggplot2 (with no labels)

Question

Below is example code of a plot that does almost exactly what I want. The only thing I want to add is tick marks on the x axis (same size as the major ticks) according to the minor_breaks defined below.

df <- data.frame(x = c(1900,1950,2000), y = c(50,75,60))

p <- ggplot(df, aes(x=x, y=y))
  p + geom_line() + 
  scale_x_continuous(minor_breaks = seq(1900,2000,by=10), breaks = seq(1900,2000,by=50), limits = c(1900,2000), expand = c(0,0)) +
  scale_y_continuous(breaks = c(20,40,60,80), limits = c(0,100)) +
  theme(legend.position="none", panel.background = element_blank(), 
  axis.line = element_line(color='black'), panel.grid.minor = element_blank())

Thanks in advance, --JT

Answer 1

This would do it in the precise instance:

scale_x_continuous(breaks= seq(1900,2000,by=10), 
                  labels = c(1900, rep("",4), 1950, rep("",4), 2000), 
                  limits = c(1900,2000), expand = c(0,0)) +

Here's a function that is not bullet-proof but works to insert blank labels when the beginning and ending major labels are aligned with the start and stopping values for the at argument:

insert_minor <- function(major_labs, n_minor) {labs <- 
                              c( sapply( major_labs, function(x) c(x, rep("", 4) ) ) )
                              labs[1:(length(labs)-n_minor)]}

Test:

p <- ggplot(df, aes(x=x, y=y))
  p + geom_line() + 
  scale_x_continuous(breaks= seq(1900,2000,by=10), 
                     labels = insert_minor( seq(1900, 2000, by=50), 4 ), 
                     limits = c(1900,2000), expand = c(0,0)) +
  scale_y_continuous(breaks = c(20,40,60,80), limits = c(0,100)) +
  theme(legend.position="none", panel.background = element_blank(), 
        axis.line = element_line(color='black'), panel.grid.minor = element_blank())