title: How do i merge two dataframes in R but keep all missing values.
body: <p>I need to combine to data frames that have different lengths, and keep all the "missing values". The problem is that there are not really missing values, but rather just less of one value than another. </p>
  
  <p>Example:
  df1 looks like this:</p>
  
  <pre><code>Shrub value period
  1     0.5   1  
  2     0.6   1
  3     0.7   1
  4     0.8   1 
  5     0.9   1
  10    0.9   1
  1     0.4   2
  5     0.4   2
  6     0.5   2
  7     0.3   2
  2     0.4   3
  3     0.1   3
  8     0.5   3
  9     0.2   3
  </code></pre>
  
  <p>df2 looks like this:</p>
  
  <pre><code>Shrub   x    y
  1       5    8
  2       6    7
  3       3    2
  4       1    2
  5       4    6 
  6       5    9
  7       9    4 
  8       2    1   
  9       4    3
  10      3    6
  </code></pre>
  
  <p>and i want the combined dataframe to look like:</p>
  
  <pre><code>Shrub   x    y    value   period
  1       5    8    0.5        1
  2       6    7    0.6        1
  3       3    2    0.7        1
  4       1    2    0.8        1
  5       4    6    0.9        1 
  6       5    9    NA         1
  7       9    4    NA         1 
  8       2    1    NA         1   
  9       4    3    NA         1
  10      3    6    0.9        1
  1       5    8    0.4        2
  2       6    7    NA         2
  3       3    2    NA         2
  4       1    2    NA         2
  5       4    6    0.4        2
  6       5    9    0.5        2
  7       9    4    0.3        2
  8       2    1    NA         2
  9       4    3    NA         2
  10      3    6    NA         2
  1       5    8    NA         3
  2       6    7    0.4        3
  3       3    2    0.1        3
  4       1    2    NA         3
  5       4    6    NA         3
  6       5    9    NA         3
  7       9    4    NA         3 
  8       2    1    0.5        3   
  9       4    3    0.2        3
  10      3    6    NA         3
  </code></pre>
  
  <p>I have tried the merge command using all = TRUE, but this does not give me what i want. I haven't been able to find this anywhere so any help is appreciated! </p>