function push( datum ) {
	/* eslint-disable no-invalid-this */
	debug( 'Current value: %s.', JSON.stringify( this._data ) );
	if ( this._data.length >= this._bufferSize ) {
		this._data.shift();
	}
	this._data.push( datum );

	debug( 'New value: %s.', JSON.stringify( this._data ) );
	this.emit( 'change' );

	return this;
}
Appends data.

@private
@param {*} datum - data to append
@returns {Sparkline} class instance