Ok, first of all: Non-existing values are evaluated to null (roughly translates to "nada", "nothing", "not there"), which isn't equal to 0, which is a well defined value.
MongoDB has no semantical understanding of the difference between 0 and 42, for example. So how should MongoDB decide which value to assume for a day in the time (of which mongo has no semantical understanding, too)?
Basically, you have two choices: save a 0 for each day when no value is to record or you iterate in your app over the days in the time you want to create a chart for and issue 0 for each day no value exists as a substitute. Id' suggest doing the former, since that would make it possible using the aggregation framework.
