C3js - Grid Rows Displaying Over Content

I am trying to make x-axis grid lines behind the content of a c3js graphical chart.

I was playing around with z-index which didn't work. I tried with opacity which didn't work either.

Here is a JSFiddle with the code I used.

https://jsfiddle.net/chaitanya81/rvhb0fy4/1/

var chart = c3.generate({
        bindto: '#chart',
        data: {
            x : 'x',
            columns: [
                ['x', 'M','T','W','TH','F','SA','SU'],
                ['revenue', 200, 300, 200, 400, 500, 700, 600.56]
            ],
            type: 'bar'
        },
        color: {
            pattern: ["#ff9900"]
        },
        axis: {
            x: {
                type: 'category', // this needed to load string x value
                tick: {
                    outer: false
                }
            },
            y: {
                tick: {
                    outer: false
                }
            }
        },
        grid: {
            x: {
                lines: [
                    {value: "M"},
                    {value: "T"},
                    {value: "W"},
                    {value: "TH"},
                    {value: "F"},
                    {value: "SA"},
                    {value: "SU"}
                ]
            }
        },
        bar: {
            width: {
                ratio: 0.4
            }
        },
        legend: {
            hide: true
        },
        tooltip: {
            contents: function (data, defaultTitleFormat, defaultValueFormat, color) {
                    var $$ = this, config = $$.config,
                        titleFormat = config.tooltip_format_title || defaultTitleFormat,
                        nameFormat = config.tooltip_format_name || function (name) { return name; },
                        valueFormat = config.tooltip_format_value || defaultValueFormat,
                        text, i, title, value;

                        for (i = 0; i < data.length; i++) {
                            if (! (data[i] && (data[i].value || data[i].value === 0))) { continue; }

                            if (! text) {
                              title = titleFormat ? titleFormat(data[i].x) : data[i].x;
                              text = "<div id='tooltip' class='d3-tip'>";
                            }
                            value = valueFormat(data[i].value, data[i].ratio, data[i].id, data[i].index);
                            text += "<span class='value'>$" + value + "</span>";
                            text += "</div>";
                        }

                    return text;
            }
        },
        transition: {
            duration: 1000
        }
});

      

        
        
        
      

    

Has anyone tried this with c3js charts?

Thanks in Advance.