title: Hide/Drop missing values in heat map with ggplot2
body: <p>I have a data frame with continous missing values from 11 Jan to 14 Jan 2016 as</p>
  
  <pre><code> library(lubridate)
      set.seed(123)
      timestamp1 &lt;- seq(as.POSIXct("2016-01-01"),as.POSIXct("2016-01-10 23:59:59"), by = "hour")
      timestamp2 &lt;- seq(as.POSIXct("2016-01-15"),as.POSIXct("2016-01-20 23:59:59"), by = "hour")
      data_obj &lt;- data.frame(value = c (rnorm(length(timestamp1),150,5),rnorm(length(timestamp2),110,3)),timestamp = c(timestamp1,timestamp2))  
     data_obj$day &lt;- lubridate::date(data_obj$timestamp)
     data_obj$hour &lt;- lubridate::hour(data_obj$timestamp)
  </code></pre>
  
  <p>When I plot a heat map using</p>
  
  <pre><code>ggplot(data_obj,aes(day,hour,fill=value)) + geom_tile()
  </code></pre>
  
  <p>I get heat map like below one; red marked rectangular region corresponds to missing values</p>
  
  <p><a href="https://i.stack.imgur.com/CPHJa.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CPHJa.png" alt="enter image description here"></a></p>
  
  <p>How should I entirely hide this blank area and make a continuous heat map?</p>
  
  <p><strong>Note that I do not want to change the format of x-axis date and I don't want to show missing values with some other color</strong>.</p>