title: Chart monthly average NDVI values
body: <p>I am hoping to obtain monthly averages for NDVI derived from Landsat data and then produce a line chart. </p>
  
  <p>I have succeeded in cloudmasking Landsat 8 and adding an NDVI band. </p>
  
  <p>Working with code suggested in another post I was able to create an image collection with one NDVI value per month for 5 years.</p>
  
  <p>Now however, when I try and chart this image collection there is no "system:time_start" available so I cannot produce a time series. </p>
  
  <p>Any help would be appreciated. 
  Maybe I have to go about calculating the monthly mean NDVI values in a different way. </p>
  
  <p>My code is below. </p>
  
  <p><a href="https://code.earthengine.google.com/7c0abe5c696a4aa246647f4b34e2f48c" rel="nofollow noreferrer">https://code.earthengine.google.com/7c0abe5c696a4aa246647f4b34e2f48c</a></p>