In Sale Report Y-axis add amount with format using the below code:-

\addons\web_graph\static\src\js\graph.js

    make_graph: function (mode, container, data) {
        if (mode === 'area') { mode = 'line'; }
        var format = this.get_format(this['options_' + mode](data));        
        /* ADD Y axis Amount in Sale Report  */
         if (this.dataset.model == 'sale.report')           
         xy_value = data.data[0]['data'][0]; //xy_value contains x, y axis value 0, 10200 
         var str = xy_value.toString();  // Object value convert into string
         var n = str.substr(str.indexOf(",") + 1); // Get aftercomma value
          {
            //format['xaxis']['title'] = 'x-axis title';
            format['yaxis']['title'] =  parseInt(n).toLocaleString('en-US', {minimumFractionDigits: 2}); //Format value
        }
        /* End */
        return Flotr.draw(container, data.data, format);
    },