end FUNCTION ascending() 
FUNCTION: quartiles( data )
Computes quartiles for a dataset array.

@private
@param {array} data - sorted (ascending) array
@returns {array} quartiles as a 2-element array
function quartiles( data ) {
		var qValues = new Array( 2 ),
			vector;

		vector = new Array( data.length );
		for ( var i = 0; i < data.length; i++ ) {
			vector[ i ] = data[ i ][ 1 ];
		}

		qValues[ 0 ] = quantile( vector, 0.25 );
		qValues[ 1 ] = quantile( vector, 0.75 );
		
		return qValues;
	}