correct error when a pie starts empty

Fix: tween callbacks run after chart is destroyed.

- This causes some issues when unloading arc type charts

And update the associated assert.

By default it starts at 'M-180', IMO asserting that it then starts
at 'M180' is sufficient to ensure that we passed 'Math.PI/2' instead of
'-1 * Math.PI/2'

So let's always use this method for pie/donut to simplify code path.

There are way less cache miss when we take hiddenTargetId into account and
clear the cache when legend is toggled than not doing this.

Can be configured using:
{
  data: {
    stack: {
      normalize: true
    }
  }
}

Defaults to false.

To works the data must be grouped.

Note that SVGRect, returned by getBBox, only contains {x,y,width,height}
and not {top,left}.

* add pie.padAngle and donut.padAngle options

* add API to get/set padAngle for both pie and donut

* add pie.padAngle and donut.padAngle in reference doc

* add padAngle option in donut sample

* bind getPadAngle method to self

* use chart main instead of global D3 to select chart's elements

Bumps [sprockets](http://getsprockets.org/

) from 2.12.4 to 2.12.5.

Signed-off-by: dependabot[bot] <support@github.com>

Bumps [rack](https://github.com/rack/rack) from 1.6.8 to 1.6.11.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rack/rack/compare/1.6.8...1.6.11

)

Signed-off-by: dependabot[bot] <support@github.com>

Bumps [ffi](https://github.com/ffi/ffi) from 1.9.18 to 1.11.1.
- [Release notes](https://github.com/ffi/ffi/releases)
- [Changelog](https://github.com/ffi/ffi/blob/master/CHANGELOG.md)
- [Commits](https://github.com/ffi/ffi/compare/1.9.18...1.11.1

)

Signed-off-by: dependabot[bot] <support@github.com>

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.7.2 to 1.10.3.
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sparklemotion/nokogiri/compare/v1.7.2...v1.10.3

)

Signed-off-by: dependabot[bot] <support@github.com>

* chore(package): update rollup to version 1.19.2

* chore(package): update lockfile package-lock.json

* fix positioning of the X Axis label

* fix position for xAxis label demo

Only unload when loading if unload arg is truthy

add test to ensure no negative offset on long centered title