make hist more flexible

[SimonDanisch]

With scale_to, offset, fillto and flip, one can put multiple histograms into the same plot:

fig = Figure()
ax = Axis(fig[1, 1])
for i in 1:5
     hist!(ax, randn(1000), scale_to=0.6, offset=i, fillto=i, direction=:x, flip=true)
end
fig

[SimonDanisch]

@piever @jkrumbiegel do you have any concerns about this, or can we merge?

[piever]

Nice! I've added a couple of comments, based on the observation that the API should try to respect as much as possible the proportionality between bar length and number of entries in bin.

[jkrumbiegel]

We wanted to find a consistent "interface" for these flipping keywords. (Also because AlgebraOfGraphics should be able to use this information). We currently have direction and orientation, this would add flip.

[piever]

We currently have direction and orientation, this would add flip.

IIUC, flip here is not x vs y, but rather "bars go towards the right vs left (or down vs up)". That being said, if we wanted to use flip for "flip x and y axis" in the future, this use here would conflict.

[SimonDanisch]

Alright, I removed flip since one can just do that via scale_to=-x.
I also moved offset to the barplot recipe, and made it set fillto if fillto is automatic.
So now the example is a bit simpler:

fig = Figure()
ax = Axis(fig[1, 1])
for i in 1:5
     hist!(ax, randn(1000), scale_to=-0.6, offset=i, direction=:x)
end
fig

[piever]

LGTM! I've just added a small comment to disable offset when stack is also given. We may want to implement offset + stack in the future, but that doesn't need to happen in this PR.