title: Error: Invalid input: date_trans works with objects of class Date only
body: <p>I have a time series dataset called "Data" which contains water elevation data for several wells over many years of sampling dates. The head of the data.frame looks like this:</p>
  
  <pre><code>           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
  4    2002-06-05            MW-3 929.36
  5    2002-06-12            MW-3     NA
  6    2002-06-13            MW-3 929.47
  7    2002-06-19            MW-3 929.42
  8    2002-06-26            MW-3 930.02
  9    2002-07-05            MW-3 930.00
  </code></pre>
  
  <p>I am trying to use ggplot to plot water elevation over time for each well, such that my x-axis is "Date," my y-axis is "Elev" and each well is plotted in a different color. I have created this plot with the code below, and it is to my satisfaction. </p>
  
  <p>My problem is that I am trying to overlay gray rectangles with geom_rect to show the periods in which a well pump was on. I think I am very close, but I must be doing something wrong with date formatting (?), because I keep getting the following error: </p>
  
  <blockquote>
    <p>Error: Invalid input: date_trans works with objects of class Date only</p>
  </blockquote>
  
  <p>Any help? Thanks in advance!</p>
  
  <p>Here is my code:</p>
  
  <pre><code>#Import and fix up data
  Data = read.csv("water_elevation_for_R_date.csv", stringsAsFactors=FALSE)
  colnames(Data)[1] &lt;- "Date"
  Data$Date = as.Date(Data$Date, format = "%m/%d/%Y")
  Data$Well &lt;- as.factor(Data$Well)
  Data$Elev &lt;- as.numeric(Data$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 = "2004-04-29", 
                xmax = "2004-12-20",
                ymin = -Inf, 
                ymax = Inf),
            fill = "gray", 
            alpha = 0.5)
  </code></pre>