Below script allow us to plot a static target line in a chart.
Supported chart types : Bar chart, Column chart, Line chart, Area chart

widget.on('processresult', function(se, ev){
ev.result.yAxis[0].plotLines = [{
color: '#2ec7b5',
dashStyle: 'LongDash',
width: 4,
value: 800,
zIndex: 5,
label : {
text : 'Target'
}
}]
})
Possible values of dashStyle are :
'Solid',
'ShortDash',
'ShortDot',
'ShortDashDot',
'ShortDashDotDot',
'Dot',
'Dash',
'LongDash',
'DashDot',
'LongDashDot',
'LongDashDotDot'