I do not have the response but in my opinion, the behaviour you describe is what i would expect because "no data" or "empty data" is not the same as "0 value".
The actual display, do show the lack of information for this day.
That said, if you need to transform your empty value to 0, then i think it would be is a data cleaning job that must be done before the analysis. For example, in the database by defining a 0 default value. Or with your ETL tool (read Talend), or in your Excel imported file, etc...