When you give the path to your URL, make sure to use the require.toUrl method.
For example, if your file is located here :
bundles\my\custom\ext\chart\chart_viz_ext_chart-src\myfile.xml
do
require.toUrl("chart_viz_ext_chart-src\myfile.xml");
Do not forget to add require to your dependencies.
Doc there : Visualization Extension Plugin for SAP Web IDE Guide
For instance :
define(... [ "require", ... ], function(require ...) { }