fill missing dates with other values than zero

Is your feature request related to a problem? Please describe. When executing resultset.series(), fillMissingDates is true by default, and the values of the missing dates are filled with 0. In some scenarios, I have a data with 0 as values, and it makes it hard to identify which is actual date and which is missed date in the result. An ability to set the missing date's value to null or to -1 it could be useful to some charts such as nivo (in nivo value: null is considered empty)

Describe the solution you'd like

1. Add another option missingDateValue
2. fillMissingDate will be either boolean or an object describing { enabled: boolean, value: number | null | etc.. }