Chandni

You just need to extend the proper widget class using include, like:

var GraphWidget = require('web.GraphWidget');
GraphWidget.include({
    display_bar: function () {
        // prepare data for bar chart
        var data, values,
            measure = this.fields[this.measure].string;

        // zero groupbys
        if (this.groupbys.length === 0) {
            data = [{
                values: [{
                    x: measure,
                    y: this.data[0].value}],
                key: measure
            }];
        } 
        // one groupby
        if (this.groupbys.length === 1) {
            values = this.data.map(function (datapt) {
                return {x: datapt.labels, y: datapt.value};
            });
            data = [
                {
                    values: values,
                    key: measure,
                }
            ];
        }
        if (this.groupbys.length > 1) {
            var xlabels = [],
                series = [],
                label, serie, value;
            values = {};
            for (var i = 0; i < this.data.length; i++) {
                label = this.data[i].labels[0];
                serie = this.data[i].labels[1];
                value = this.data[i].value;
                if ((!xlabels.length) || (xlabels[xlabels.length-1] !== label)) {
                    xlabels.push(label);
                }
                series.push(this.data[i].labels[1]);
                if (!(serie in values)) {values[serie] = {};}
                values[serie][label] = this.data[i].value;
            }
            series = _.uniq(series);
            data = [];
            var current_serie, j;
            for (i = 0; i < series.length; i++) {
                current_serie = {values: [], key: series[i]};
                for (j = 0; j < xlabels.length; j++) {
                    current_serie.values.push({
                        x: xlabels[j],
                        y: values[series[i]][xlabels[j]] || 0,
                    });
                }
                data.push(current_serie);
            }
        }
        var svg = d3.select(this.$el[0]).append('svg');
        svg.datum(data);

        svg.transition().duration(0);

        var chart = nv.models.multiBarChart();
        var maxVal = _.max(values, function(v) {return v.y})
        chart.options({
          margin: {left: 12 * String(maxVal && maxVal.y || 0).length},
          delay: 250,
          transition: 10,
          showLegend: _.size(data) <= MAX_LEGEND_LENGTH,
          showXAxis: true,
          showYAxis: true,
          rightAlignYAxis: false,
          stacked: this.stacked,
          reduceXTicks: false,
          rotateLabels: 40,
          showControls: (this.groupbys.length > 1)
        });
        chart.yAxis.tickFormat(function(d) { return formats.format_value(d, { type : 'float' });});

        chart(svg);
        this.to_remove = chart.update;
        nv.utils.onWindowResize(chart.update);
    }
});

Hope this help you