Averange values every 900 seconds(15 minutes)

Hi, I must take the values and make the average of their, every 900 seconds. So a unique value every 15 minutes, how can I do?.
This is my begin code:

Misurazioni Potenza

Sensori Web Server

Misurazioni Potenza

    <select id="foo">
    <option value="">Scegli una misurazione</option>
    <option value="http://localhost/plotquery1.php">Misurazione Potenza</option>
    <option value="http://localhost/plotquery2.php">Misurazione Temperatura e Umidita'</option>

    document.getElementById("foo").onchange = function() {
        if (this.selectedIndex!==0) {
            window.location.href = this.value;
      <div id="graph"></div>
        var layout = 
          range: ['2017-11-01 08:30:00','2017-11-31 23:00:00' ]
          title: 'kWh',
           family: 'Courier New, monospace',
           size: 18,
           color: '#7f7f7f'
          range: [0, 2]
        Plotly.plot('graph',[{"x":["2017-11-12 13:10:33.669715","2017-11-12 13:10:39.162448","2017-11-12 13:10:44.077944",...],y":["1.386","1.386","1.386","1.188","1.188","1.386","1.188","1.188,...]}, layout);

So I haven’t put all values (they are too many), but the format timestamp is this years-month-day hh:mm:ss:… Suggests?

We added aggregation transforms a few months ago without much advertisement.

Here’s a codePen to help get started: https://codepen.io/etpinard/pen/VrzoZb?editors=0010

Let me know if you help any questions.