function set( fcn ) {
	/* eslint-disable no-invalid-this */
	if ( !isFunction( fcn ) ) {
		throw new TypeError( format( 'invalid assignment. `%s` must be a function. Value: `%s`.', 'xScale', fcn ) );
	}
	debug( 'Current value: %s.', this._xScale );

	this._xScale = fcn;
	debug( 'New Value: %s.', this._xScale );

	this.emit( 'change' );
}
Sets the x-scale function.

@private
@param {Function} fcn - scale
@throws {TypeError} must be a function