var eventStream = require( 'event-stream' ),
    cStream = require( 'flow-covariance' );

// Create some data... (negatively corrrelated)
var data = new Array( 1000 ), value;
for ( var i = 0; i < data.length; i++ ) {
    value = Math.random() * 50;
    data[ i ] = {
        'y1': 50 + value,
        'y2': 50 - value
    }
}

// Create a readable stream:
var readStream = eventStream.readArray( data );

// Create a new stream:
var stream = cStream()
    .accessors( 'y1', function( d ) {
        return d.y1;
    })
    .accessors( 'y2', function( d ) {
        return d.y2;
    })
    .stream();

// Create a pipeline:
readStream.pipe( stream )
    .pipe( eventStream.stringify() )
    .pipe( process.stdout );