message: point to correct crossing axis
diff: @@ -2409,13 +2409,13 @@ var PptxGenJS = function(){
  if ( !rel.opts.valAxes || rel.opts.valAxes.length !== rel.opts.catAxes.length ) {
  throw new Error('There must be the same number of value and category axes.');
  }
  - strXml += makeCatAxis(mix(rel.opts, rel.opts.catAxes[0]), AXIS_ID_CATEGORY_PRIMARY);
  + strXml += makeCatAxis(mix(rel.opts, rel.opts.catAxes[0]), AXIS_ID_CATEGORY_PRIMARY, AXIS_ID_VALUE_PRIMARY);
  if ( rel.opts.catAxes[1] ) {
  - strXml += makeCatAxis(mix(rel.opts, rel.opts.catAxes[1]), AXIS_ID_CATEGORY_SECONDARY);
  + strXml += makeCatAxis(mix(rel.opts, rel.opts.catAxes[1]), AXIS_ID_CATEGORY_SECONDARY, AXIS_ID_VALUE_PRIMARY);
  }
  }
  else {
  - strXml += makeCatAxis(rel.opts, AXIS_ID_CATEGORY_PRIMARY);
  + strXml += makeCatAxis(rel.opts, AXIS_ID_CATEGORY_PRIMARY, AXIS_ID_VALUE_PRIMARY);
  }
  rel.opts.hasArea = hasArea(rel.opts.type);
  @@ -3051,7 +3051,7 @@ var PptxGenJS = function(){
  return strXml;
  }
  - function makeCatAxis(opts, axisId) {
  + function makeCatAxis(opts, axisId, valueAxisId) {
  var strXml = '';
  // Build cat axis tag
  @@ -3098,7 +3098,7 @@ var PptxGenJS = function(){
  strXml += ' <a:endParaRPr lang="'+ (opts.lang || 'en-US') +'"/>';
  strXml += ' </a:p>';
  strXml += ' </c:txPr>';
  - strXml += ' <c:crossAx val="'+ axisId +'"/>';
  + strXml += ' <c:crossAx val="'+ valueAxisId +'"/>';
  strXml += ' <c:crosses val="autoZero"/>';
  strXml += ' <c:auto val="1"/>';
  strXml += ' <c:lblAlgn val="ctr"/>';
  