Error: Invalid input: date_trans works with objects of class Date only

The problem seems to be in your geom_rect area (it plots without this). Other “date_trans” errors on this site point to needed to set dates with as.Date. So yes, you were in the right debug area. This works:

Wrap your minimum and maximum in xmin and xmax call in geom_rect section:

aes(xmin = as.Date("2004-04-29", "%Y-%m-%d"), 
    xmax = as.Date("2004-12-20",  "%Y-%m-%d"),

CODE BELOW FOR OTHERS TO USE

I created three data lines only as @YourEconProf suggested.

#Import and fix up data
#Data = read.csv("water_elevation_for_R_date.csv", stringsAsFactors=FALSE)
#Date            Well   Elev
#1    2002-05-23            MW-3 929.04
#2    2002-05-29            MW-3 929.39
#3    2002-05-31            MW-3 929.37
# etc...
Data = data.frame(Date = c(as.Date("2002-05-23", "%Y-%m-%d"),
                           as.Date("2002-05-29", "%Y-%m-%d"),
                           as.Date("2002-05-31", "%Y-%m-%d")),
                  Well = c("MW-3","MW-3","MW-3"),
                  Elev = c(929.04, 929.39, 929.37))

colnames(Data)[1] <- "Date"
Data$Date = as.Date(Data$Date, format = "%m/%d/%Y")
Data$Well <- as.factor(Data$Well)
Data$Elev <- as.numeric(Data$Elev)

#Load ggplot and scales
library(ggplot2)
library(scales)

#Create graph
ggplot(data= Data, aes(x = Date, y = Elev, group = Well, colour = Well)) +
  geom_line(size = 0.75) +
  xlab("") + ylab("Elevation (ft.)") +
  scale_color_brewer(palette = "Spectral") +
  scale_x_date(breaks = date_breaks("2 year"),
               labels = date_format("%Y")) +
  theme_bw()+
  theme(plot.background = element_blank(), 
        panel.grid.major = element_blank(), 
        panel.grid.minor = element_blank(), 
        panel.border = element_blank(), 
        axis.line.x = element_line(color = "black"),
        axis.line.y = element_line(color = "black")) +
  geom_rect(data = Data, 
            aes(xmin = as.Date("2004-04-29", "%Y-%m-%d"), 
                xmax = as.Date("2004-12-20",  "%Y-%m-%d"),
                ymin = -Inf, 
                ymax = Inf),
            fill = "gray", 
            alpha = 0.5)

Which gets you this: