I searched the whole code and I didn't see anything related to interpolation. I also made a plunker so you can see it in real time.
http://plnkr.co/edit/5p00VXhAnMn6gSoaPJv0?p=preview -- You received this message because you are subscribed to the Google Groups "Google Visualization API" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/google-visualization-api. For more options, visit https://groups.google.com/d/optout.
