@mhdajmalik,
You can set indexLabelLineThickness to show lines drawn from dataPoint to the indexLabel, which defaults to 0 in every chart-type except pie/doughnut. Please take a look at this jsfiddle.
If you specifically looking to show lines downwards, you can do so with couple of lines of code. Please take a look at this jsfiddle.
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Vishwas R
Team CanvasJS