title: How to set index and key when creating tsibble from existing data?
body: <p><strong>Problem</strong>
  I want to create a tsibble but it's unclear how to set the key and how to set the index. Not a tibble.</p>
  <p>I tried the following:</p>
  <pre><code>ts &lt;- t %&gt;%
    as_tsibble(
      key = t$key_field, 
      index = c(t$date, to$state), 
      regular = FALSE
    )
  </code></pre>
  <p>But got the error:</p>
  <pre><code>Error: Can't subset columns that don't exist.
  x Columns `AA`, `AA`, `AA`, `AA`, =etc. don't exist.
  
  There is NO columns named 'AA', this `AA`, `AA`, `AA`, `AA` is actually the data!
  </code></pre>
  <p><strong>Example Needed</strong>
  Would appreciate a real world tsibble created from data collection from another existing data.frame source.</p>
  <p><strong>Discussion Needed</strong>
  Discussion on creating a tsibble and define an index, key using existing data. The code I provided complains of</p>
  <p>** Data **</p>
  <p>Region (chr), State (chr), Purpose (chr), Trips (dbl), Year (dbl)</p>
  <pre><code>1998 Q1 Adelaide    South Australia Business    135.0776903 1998
  1998 Q2 Adelaide    South Australia Business    109.9873160 1998
  1998 Q3 Adelaide    South Australia Business    166.0346866 1998
  1998 Q4 Adelaide    South Australia Business    127.1604643 1998
  1999 Q1 Adelaide    South Australia Business    137.4485333 1999
  1999 Q2 Adelaide    South Australia Business    199.9125861 1999
  1999 Q3 Adelaide    South Australia Business    169.3550898 1999
  1999 Q4 Adelaide    South Australia Business    134.3579372 1999
  2000 Q1 Adelaide    South Australia Business    154.0343980 2000
  2000 Q2 Adelaide    South Australia Business    168.7763637 2000
  2005 Q3 Australia's Coral Coast Western Australia   Business    28.6365371  2005
  2005 Q4 Australia's Coral Coast Western Australia   Business    26.4668880  2005
  2006 Q1 Australia's Coral Coast Western Australia   Business    19.0804140  2006
  2006 Q2 Australia's Coral Coast Western Australia   Business    25.8851570  2006
  2006 Q3 Australia's Coral Coast Western Australia   Business    35.5701650  2006
  2006 Q4 Australia's Coral Coast Western Australia   Business    16.8853340  2006
  2007 Q1 Australia's Coral Coast Western Australia   Business    34.5039748  2007
  2007 Q2 Australia's Coral Coast Western Australia   Business    21.6070762  2007
  2007 Q3 Australia's Coral Coast Western Australia   Business    38.6497565  2007
  2007 Q4 Australia's Coral Coast Western Australia   Business    26.0811098  2007
  </code></pre>
  <p>Here's another attempt with a different error:</p>
  <p><a href="https://i.stack.imgur.com/BvUw9.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/BvUw9.png" alt="TSibble execution for index, key, error message" /></a></p>