Fixing chart.unlayer (removing old refernece to internal layer array) and adding chart.relayer. Added tests to both.
Addressing @jugglinmike's comments: Moving tests to integration.js, removing _chart reference from layer, removing 'relayer' in favor of layering.
removing function definition test
Adding verification that upon layer reattaching, the second argument is in fact a layer.
Release v0.1.3
Update git URL in package.json.
Support running tests from the command line
Incorporate TravisCI
Fix layer event to match documentation (chain).

* spec chaining for chart events
* spec chaining for layer events
* implement chaining for layer events.
Configure JSHint to warn about unused variables

...and remove unused variables from test files.
Disallow single quotes and trailing whitespace

...and update the tests to conform.
Remove unused variable

Simplify implementation of `Chart.prototype.mixin` by removing unnecessary explicit handling of selection.
Improve build process

Programatically wrap the concatenated source code in an
immediately-invoked function expression at build time. This allows for
easier code sharing between files, lessening the friction in dispersing
the source across multiple files.
Add sourcemap support
Break API definitions into separate files
Fix whitespace

Now that the source files are programatically wrapped in an
immediately-invoked function expression at build time, the base-line
indentation should be removed.
Organize example charts
Honor inherited implementations of Chart#transform

Chart#extend and Chart#mixin are intended to help authors create new
chart definitions from existing constructors. These new definitions may
define their own data transformation logic, but they likely expect the
previously-defined transformations to take place as well (which is why
they have chosen to reuse the "base" chart).
Limit Chart options to a single parameter

Supporting a variable number of chart options needlessly complicates the
internals of d3.chart.
Simplify `Chart#mixin`

In an effort to honor the single responsibility principle, modify
`Chart#mixin` to operate with previously-created Chart instances.
Additionally, require a unique string name for each mixin.