Map missing values using a tile plot

MissingMap

# Usage
#  MissingMap(df, date.column, var.column)
#  df = data.frame that contains a column of dates and column of variables that may have missing values
#  date.column = quoted name of the column cotaining dates
#  var.column = quoted name of the column containing the variable
#
# Produces a tile plot showing here missing values occur in the sequence of dates
# https://tonyladson.wordpress.com/2014/12/21/mapping-missing-data/

MissingMap <- function(df, date.column, var.column){ 
  
  require(lubridate)
  require(ggplot2)
  
  # labels for x axis
  month.start <- yday(seq(as.Date('2010-01-01'),as.Date('2010-12-31'), by='months'))
  
  # tile values
  missing.factor <- factor(is.na(df[[var.column]]), labels = c('not missing','missing'))
  
  # assemble data frame to pass to ggplot
  df <- data.frame(x.day = yday(df[[date.column]]), 
                   y.year = factor(year(df[[date.column]])),
                   var = df[[var.column]], missing.factor = missing.factor)
   
    
  pl<- ggplot(data=df, aes(x.day, y.year))
   
  pl + geom_tile(aes(fill = missing.factor)) + 
   scale_fill_manual(values = c('gray','red'), name='' ) + 
   theme_bw() +
   theme(axis.title.x = element_text(colour="grey20", size=20, vjust=-2)) +
   theme(axis.text.x = element_text(colour="grey20",size=12)) +
   theme(axis.title.y = element_text(colour="grey20",size=20, vjust= 1)) +
   theme(axis.text.y = element_text(colour="grey20",size=12)) +
   theme(legend.title = element_text(colour="grey20",size=12)) +
   theme(plot.margin = unit(c(2.5, 2.5, 2.5, 2.5), "cm")) +
   ylab("Year") +
   scale_x_continuous(name="Month", breaks=month.start, labels=month.abb)
}