Accepted answer

Updated your plunk. The chart is drawn in a sub-group that is translated. You therefore need to add your line to that group (what I did), or translate your line of the same amount (harder for synchronization):

var line ='.nv-lineChart').append('line')
            .attr('x1', chart.xAxis.scale()(0))
            .attr('x2', chart.xAxis.scale()(100))
            .attr('y1', chart.yAxis.scale()(4.5))
            .attr('y2', chart.yAxis.scale()(4.5)) 
            .attr('stroke-width', 2)
            .attr('stroke', 'red') 

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