Change to a 24 hour format for datetime data in Google Charts

You need to format the datetimes using a DateFormatter.

// format dates
// ex: "August 5, 2013 1:45 PM" formatted as "05/08/2013 13:45"
var dateFormatter = new google.visualization.DateFormat({pattern: 'dd/MM/yyyy HH:mm'});
dateFormatter.format(data, 0);

You can format the axis labels by setting the hAxis.format option:

var options = {
    hAxis: {
        format: 'dd/MM/yyyy HH:mm'
    }
    title: 'price'
};

The date formats support most of the ISO date formatting patterns.