May I ask for further illumination of my dumbness?

I had been merrily plotting ..density.. and using xlim() to constrain the
x-axis like this and everything was working fine:

df<-data.frame(names=c("Bob","Mary","Joe","Bob","Bob"))
ggplot(df,aes(names,..density..,group=1))+geom_histogram()+xlim("Bob","Mary")

So I was a little surprised when this didn't work the same way:

ggplot(df,aes(names))+geom_histogram()+xlim("Bob","Mary")

Actually, looking at the above sparks an idea.  I thought there was a
difference between plotting ..count.. and ..density.., but in fact it's the
group=1 that makes the first example work.  Both of these behave as one
would hope:

ggplot(df,aes(names,group=1))+geom_histogram()+xlim("Bob","Mary")
ggplot(df,aes(names,..count..,group=1))+geom_histogram()+xlim("Bob","Mary")

...though I admit that I'm not exactly sure why.  Can anyone explain?

Thanks,

-J

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