title: how to draw line chart with date fomat on excel using openpyxl of python
body: <p>I draw a line chart on excel using openpyxl </p>
  
  <p>In excel sheet:</p>
  
  <pre><code>     date     value
  "2015-05-01" | 0.1
  "2015-05-02" | 0.2
  "2015-05-03" | 0.3
  "2015-05-04" | 0.4
  </code></pre>
  
  <p>But it gives integer format for date data like "45612"... How could I draw line chart with date format? (date is x-axis, and value is y-axis)</p>