Chart shows NULL as 0

I am afraid I cannot do much about that because the data integrity is handled by d3js. When D3 receives NULLs these are read as zeros. I think that if possible you should try to clean your data in the DB.
The PHP code itself is data agnostic and does not interfere with what is retrieved from the DB.

Thanks for the reply. I wanted the null values to act as an un-complete line in the line chart.
I thought there could be an easy way to do it so I made it with google charts for now which uses ‘null’ to show where the line should not be drawn.

Thanks for your feedback, unfortunately this is not something I can change as it is a behavior implemented in NVD3. In order to get the desired behavior we should implement the charts directly in D3JS. See this post for details.
https://stackoverflow.com/questions/10826180/in-d3-js-skip-append-for-null-data