fecha y dias en x

dat <- tibble::tribble(
  ~date,  ~ndvi,
  "2020-05-18", 0.7655,
  "2020-06-14",  0.723,
  "2020-07-12", 0.6178,
  "2020-08-21",  0.437,
  "2020-09-07", 0.4763,
  "2020-09-10", 0.4928,
  "2020-09-12", 0.4831,
  "2020-09-22", 0.4774,
  "2020-10-02", 0.5794,
  "2020-10-07",  0.606
) %>% 
  mutate(date = lubridate::ymd(date),  
         days = difftime(date, min(date), units="days")) 

dat %>%
  ggplot(aes(date, ndvi)) +
  geom_line() +
  scale_x_date(breaks = dat$date, 
               name = "",
               sec.axis = dup_axis(breaks=dat$date,
                                   labels = as.character(round(dat$days)),
                                   name = "Días desde aplicación"))