Collections of Stats
There are a few special OnlineStats that group together other OnlineStats: Series and Group.

Series
A Series tracks stats that should be applied to the same data stream.
y = rand(1000)
s = Series(Mean(), Variance())
fit!(s, y)Series
├─ Mean: n=1_000 | value=0.513524
└─ Variance: n=1_000 | value=0.0857702
Group
A Group tracks stats that should be applied to different data streams.
g = Group(Mean(), CountMap(Bool))
itr = zip(randn(100), rand(Bool, 100))
fit!(g, itr)Group
├─ Mean: n=100 | value=0.0737772
└─ CountMap: n=100 | value=OrderedDict{Bool, Int64}(0=>43, 1=>57)