How to modify breaks and lables within ggproto?

Question

I have a ggplot function (ggproto) to generate a manhattan plot. The code works, but I can't get my function to correctly position the breaks and labels on the x-axis. So I would really appreciate help on: How I can get my function to plot the axis that I want. Is it possible within the stat_manhattan function or would it be better to code an extra scale_x_chr(or so) function? Many thanks in advance! P.S hope my question is clear.

This is the code for the function:

StatManhattan <- ggplot2::ggproto("StatManhattan", 
                                  ggplot2::Stat,
                                  # set up parameters, e.g. unpack from list
                                  setup_params = function(data, params) {
                                    params
                                  },
                                  # Compute group is the most granular component to be called
                                  compute_group = function(data, scales, params, col_chrom) {
                                    message("My param has value ", scales)
                                    
                                    data_1 <- data %>% 
                                      dplyr::arrange(chr, pos) %>% 
                                      dplyr::transmute(x = cumsum(as.numeric(pos)), 
                                                       y,
                                                       chr)
                                    
                                    data_2 <- data_1 %>% 
                                      dplyr::distinct(chr) %>% 
                                      dplyr::mutate(colour = rep(col_chrom, length.out = dplyr::n()))
                                    
                                    # the final df
                                    data_1 %>% 
                                      dplyr::left_join(data_2, by = "chr") %>% 
                                      as.data.frame()
                                    
                                  },
                                  required_aes = c("y", "pos", "chr"),
                                  default_aes = aes(y = stat(y), 
                                                    x = stat(x), 
                                                    colour = stat(colour), 
                                                    size = 0.2)
)


stat_manhattan <- function(mapping = NULL, data = NULL, geom = "point",
                           position = "identity", na.rm = FALSE, show.legend = FALSE, 
                           inherit.aes = TRUE, col_chrom = c("magenta2", "grey60"),
                           ...) {
  ggplot2::layer(
    stat = StatManhattan, data = data, mapping = mapping, geom = geom, 
    position = position, show.legend = show.legend, inherit.aes = inherit.aes,
    params = list(na.rm = na.rm, 
                  col_chrom = col_chrom,
                  ...)
  )
}

This can generate the desired plot, but the x-axis has continuous labels - which is not what I want.

# test data
test_data <- tibble(chr = rep(1:3, each = 1000), 
                    bp  = c(sort(sample.int(1e6, 1000)), 
                            sort(sample.int(1e6, 1000)),
                            sort(sample.int(1e6, 1000))),
                    p = runif(3000, min=0, max=1))

# plotting the test data
ggplot(test_data)+aes(chr = chr, pos = bp, y =-log10(p))+stat_manhattan()

What I want to achieve is something like this, where the chromosome (chr) numbers are centered on the x-axis.

axisdf = test_data %>% 
  mutate(bp_cum = cumsum(as.numeric(bp))) %>% 
  group_by(chr) %>% 
  summarize(center=( max(bp_cum) + min(bp_cum) ) / 2 )

test_data %>% 
  mutate(bp_cum = cumsum(as.numeric(bp))) %>% 
  ggplot(., aes(x=bp_cum, y=-log10(p), colour = as.character(chr))) +
  geom_point() +
  # custom X axis:
  scale_x_continuous(label = axisdf$chr, breaks= axisdf$center )