Show days without brews in "Brews per day" statistic

[m42cel]

The "Brews per day" statistic only shows days with brews, making it seem like there are no days with 0 brews.

I think a time series graph should be continuous on the x-axis and also show days without brews. So, it's probably a "bug" by simply selecting and summing while grouping over days, which won't show days without data.

[graphefruit]

Hey @m42cel,
thanks for the ticket. The reference here would be #402.
I'd be happy to recieve a PR to fix this case, I don't have actually any capactiy to fix this kind of statistics.