[Solved]-how can i add in highcharts a different dashStyle just for a part of the graph-Highcharts

No it is not possible, you need two series, first line and second with dashed style.

You can apply 'zones' to a series: http://api.highcharts.com/highcharts/plotOptions.series.zones Just use the dashStyle option like in this example http://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/highcharts/series/color-zones-dashstyle-dot/

 zones: [{
        value: 8
    }, {
        dashStyle: 'dot'
    }]