HighChart width greater than col-sm in Bootstrap

Question

I'm working with Bootstrap and HighChart. I'm making a web page in witch I need show a plot in a tablets or desktops PC. This is my html code:

<div id="container-fluid">
 <div class="row">

   <div class="col-sm-6">
     <div class="box">
       <div id="plot1" style="width:100%; margin: 0 auto"></div>
     </div><!-- box -->
   </div><!-- col-sm-6 -->

 </div><!-- row -->
</div><!-- container-fluid -->

And this is my plot script:

$(function () {
$('#plot1').highcharts({
    title: {
        text: 'Monthly Average Temperature',
        x: -20 //center
    },
    subtitle: {
        text: 'Source: WorldClimate.com',
        x: -20
    },
    xAxis: {
        categories: ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
         'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
    },
    yAxis: {
        title: {
            text: 'Temperature (°C)'
        },
        plotLines: [{
            value: 0,
            width: 1,
            color: '#808080'
        }]
    },
    tooltip: {
        valueSuffix: '°C'
    },
    legend: {
        layout: 'vertical',
        align: 'right',
        verticalAlign: 'middle',
        borderWidth: 0
    },
    series: [{
        name: 'Tokyo',
        data: [7.0, 6.9, 9.5, 14.5, 18.2, 21.5, 25.2, 26.5, 23.3, 18.3, 13.9, 9.6]
    }, {
        name: 'New York',
        data: [-0.2, 0.8, 5.7, 11.3, 17.0, 22.0, 24.8, 24.1, 20.1, 14.1, 8.6, 2.5]
    }, {
        name: 'Berlin',
        data: [-0.9, 0.6, 3.5, 8.4, 13.5, 17.0, 18.6, 17.9, 14.3, 9.0, 3.9, 1.0]
    }, {
        name: 'London',
        data: [3.9, 4.2, 5.7, 8.5, 11.9, 15.2, 17.0, 16.6, 14.2, 10.3, 6.6, 4.8]
    }]
});

$('#plot1').highcharts().reflow();
})

The problem to I have is, the plot has a width greater than the col-sm-6 (much more, almost like a col-sm-12). Why? Thanks!