Error: Discrete value supplied to continuous variable

I am attempting to convert an original schedule to longest operating time next schedule then create waterfall plot. I am having trouble with the plot. I get an Error: Discrete vale supplied to continuous variable message. My example code appears below.

library(tidyverse)
library(ggplot2)

# original schedule of four jobs
df <- data.frame(job=c("A","B","C","D"),
                 original_begin=c("2021-01-05 07:00:00", "2021-05-01 08:30:00",
                                 "2021-05-01 10:30:00", "2021-05-01 14:00:00"),
                 original_end=c("2021-01-05 08:30:00", "2021-05-01 10:30:00",
                               "2021-05-01 14:00:00", "2021-05-01 16:00:00"))

# represent date/times as POSIXct objects
df$original_begin <- as.POSIXct(df$original_begin)
df$original_end <- as.POSIXct(df$original_end)

# calculate span, length of time each job takes
df$span <- as.numeric(difftime(df$original_end,df$original_begin))

# sort by span descending
df <- arrange(df,-span)

# assign ID now that df is correcly sorted
df$ID <- as.numeric(rownames(df))

# calculate ymin and ymax
df$ymin[1] <- min(df$original_begin)
for (i in 1:(nrow(df)-1)) {
  df$ymax[i] <- df$ymin[i] + df$span[i]
  df$ymin[i+1] <- df$ymax[i]
}
df$ymax[nrow(df)] <- df$ymin[nrow(df)] +
  df$span[nrow(df)]

# following is loosely based on tutorial found at
# https://www.stomperusa.com/2019/05/27/basic-waterfall-graphs-in-r/

# set up plot canvas, longest job first (see x=reorder(job,-span))
g <-df %>%
  ggplot(aes(x=reorder(job,-span), y=span, fill=factor(job))) +
  theme_classic() +
  theme(legend.title=element_blank())+
  theme(legend.position = "right", panel.grid = element_blank(),
        axis.text.x = element_text(angle = 90, vjust = 0.5)) +
  labs(y = "Hours", x = "Job")
g  # seems to be working as expected through here

w <- 0.5  # use to set width of bars

# attempt to create waterfall plot
g <- g +
  geom_rect(aes(xmin = ID - w/2,
                xmax = ID + w/2,
                ymin = ymin,
                ymax = ymax,
                fill = factor(job)), alpha=0.25)
g

Any assistance is appreciated


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