Tick marks (and labels) aren't aligning precisely on the upper, duplicate axis (check "0" on top) and I can't find a reason. I just have one call to the x-axis:
scale_x_continuous(expand = c(0.01, 0.01), limits = c(-550, 1000),
breaks = c(-550,-500,-400,-300,-200,-100,-50,-25, 0,25,50, 100, 200, 300, 400, 500, 750, 1000),
labels = c("", "500", "400", "300", "200","100","50","25", "0","25","50", "100", "200", "300", "400", "500", "750", "1 maf"),
sec.axis = dup_axis())
Thank you for any ideas.