How can i stop the tap event on my legend labels.

I have created a pie chart. So the problem is on tapping the legend item that slice is removed from the pie.
I dont want that to happen.

I have tried the below code

Ext.each(chart.legend.items, function(item) {
item.un("mousedown", item.events.mousedown.listeners[0].fn);
})

but it is not working for me.

please find below the code i have used

{
xtype : 'chart',
//width : Ext.os.deviceType == 'Phone' ? 310 : 800,
//height : Ext.os.deviceType == 'Phone' ? 350 : 800,
//scrollable : 'vertical',
flex : 1,
store : {
model: 'NexixViews.model.ControlViewPieChartModel',
data : []
},
shadow : true,
selSeverity : null,
legend: {
position: {
portrait: 'bottom',
landscape: 'left'
}
},

series: [
{
type : 'pie',
field : 'executions',
showInLegend: true
}
]
}