Let scales modify title

[thomasp85]

This PR is mainly justified by the units developers wish to have build in support for the units class in ggplot2. While the scale implementation will be part of ggforce (see the discussion in thomasp85/ggforce#8) some small changes are needed in ggplot2 itself in order to intercept the default scale titles before they are rendered…

[codecov-io]

Current coverage is 70.41% (diff: 100%)

Merging #1868 into master will not change coverage

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[hadley]

LGTM

I think this would also make it possible to have a scale that automatically divided by 1000, 100000 etc to make the values more readable.

That makes me wonder if the transformer should some how be involved.

[hadley]

Can you add a bullet to NEWS and add to the Scale description?

[thomasp85]

Which scale description? The man page?

I also think this opens up for some niceties in the scales such as what you describe but this is probably best left for the next release/intern

[hadley]

Yeah, the man for the base class.

Agreed that the rest can wait until the next release.

[thomasp85]

If you look at the documentation level for Scale in ggplot2-ggproto.Rd I think we can agree that a mention there would seem out of place unless the whole section gets an update...

[hadley]

Oh yeah, let's leave it for now. LGTM

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