Ensure offsetIndex and offsetgroup are not undefined

Author: my-tien

src/traces/bar/cross_trace_calc.js

@@ -352,7 +352,7 @@ function setOffsetAndWidth(gd, pa, sieve, opts) {
 
         var t = calcTrace[0].t;
         t.barwidth = barWidth;
-        t.offsetindex = trace._offsetIndex;
+        t.offsetindex = trace._offsetIndex || 0;
         t.poffset = offsetFromCenter;
         t.bargroupwidth = barGroupWidth;
         t.bardelta = minDiff;

src/traces/scatter/grouping_defaults.js

@@ -26,11 +26,12 @@ module.exports = function handleGroupingDefaults(traceIn, traceOut, fullLayout,
         };
     }
 
-    var offsetgroup = coerce('offsetgroup');
+    var offsetgroup = coerce('offsetgroup') || '';
     var offsetGroups = alignmentGroupOpts.offsetGroups;
     var offsetGroupOpts = offsetGroups[offsetgroup];
     // in barmode 'group', traces without offsetgroup receive their own offsetgroup
     // in other barmodes, traces without offsetgroup are assigned to the same offset group
+    traceOut._offsetIndex = 0;
     if(barmode !== 'group' || offsetgroup) {
         if(!offsetGroupOpts) {
             offsetGroupOpts = offsetGroups[offsetgroup] = {