How can i draw simple line in column chart in google-charts API?

I have a simple column chart. using the google-charts API:

JS code:

  function drawChart() {
    var data = google.visualization.arrayToDataTable(<?php echo $str ?>);

    var options = {
    title: 'Business Optimization Per Predicted Conversion Rate',
    bar: {groupWidth: "70%"},
    colors: ['purple','grey'],
    fontSize:20,
    legend: { position: "none" },
    width: 900,
    backgroundColor: { fill: '#efeff0'},
    };

  var chart = new google.charts.Bar(document.getElementById('chart_div_1'));
  chart.draw(data, google.charts.Bar.convertOptions(options));

My question is: How can i draw simple line here? Horizontly to the X-axis.