title: geom_raster to visualize missing values with additional colorcode
body: <p><em>This question is a follow-up to my previous question: <a href="https://stackoverflow.com/questions/57914461/adding-color-code-fill-to-vis-miss-plot">Adding color code (fill) to vis_miss plot</a></em></p>
  
  <p>I would like to visualize the "missing info" in a data frame using <code>geom_raster</code> from <code>ggplot2</code> in R while also highlighting some additional data structure using color-coding.</p>
  
  <p>Solution attempt:</p>
  
  <pre><code>library(tidyverse)
  x11()
  airquality %&gt;%
    mutate(id = row_number()) %&gt;%
    gather(-c(id,Month), key = "key", value = "val") %&gt;%
    mutate(isna = is.na(val)) %&gt;%
    mutate(Month=as.factor(ifelse(isna==TRUE,NA,Month)))  %&gt;%
    ggplot(aes(key, id, fill = Month)) +
      geom_raster() +
      labs(x = "Variable",
             y = "Row Number", title = "Missing values in rows") +
      coord_flip()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/yXcCP.png" alt="plot"></p>
  
  <p>This is almost what I want, but it would be nicer to separate the month and NA legends. Is that possible? (Note that my system does not allow me to use transparency (<code>alpha</code>)).</p>