title: timeline chart not drawing "null" data
body: <p>when I use the data below in a google sheets timeline chart it doesn't plot <code>29/04/2019 5% and 10%</code> leaving a gap in a chart, how can I resolve this? the chart should be unaffected by empty fields.</p>
  
  <pre><code>Date    5% Target   10% Target  Actual
  22/04/2019  120.6   120.6   120.6
  29/04/2019                  119.5
  22/07/2019  114.57  108.54  
  22/10/2019  108.84  97.69   
  22/01/2020  103.40  87.92   
  22/04/2020  98.23   79.13   
  22/07/2020  93.32   71.21   
  22/10/2020  88.65   64.09   
  22/01/2021  84.22   57.68   
  22/04/2021  80.01   51.91   
  22/07/2021  76.01   46.72   
  22/10/2021  72.21   42.05   
  22/01/2022  68.60   37.85   
  22/04/2022  65.17   34.06   
  22/07/2022  61.91   30.65   
  22/10/2022  58.81   27.59   
  22/01/2023  55.87   24.83   
  22/04/2023  53.08   22.35   
  22/07/2023  50.43   20.11   
  </code></pre>
  
  <p>I have tried adding another dataset and I get the error</p>
  
  <blockquote>
    <p>each values column may be followed by one or two annotation columns. column number 4 is of type date</p>
  </blockquote>
  
  <p><a href="https://docs.google.com/spreadsheets/d/1MOb1njK58ewVh1aukkhDCKrGYolgHd6lNmiS92KlNYM/edit?usp=sharing" rel="nofollow noreferrer">Here is a link to the spreadsheet</a></p>