Fixing 2409 : Calling merge on backgroundArc before setting 'd' attribute.

src/arc.js

@@ -471,9 +471,10 @@ c3_chart_internal_fn.redrawArc = function (duration, durationForExit, withTransf
     if (hasGaugeType) {
         var index = 0;
         backgroundArc = $$.arcs.select('g.' + CLASS.chartArcsBackground).selectAll('path.' + CLASS.chartArcsBackground).data($$.data.targets);
-        backgroundArc.enter().append("path")
-            .attr("class", function (d, i) { return CLASS.chartArcsBackground + ' ' + CLASS.chartArcsBackground +'-'+ i; })
-            .attr("d", function (d1) {
+        backgroundArc = backgroundArc.enter().append("path").merge(backgroundArc);
+        backgroundArc.attr("class", function (d, i) {
+            return CLASS.chartArcsBackground + ' ' + CLASS.chartArcsBackground + '-' + i;
+        }).attr("d", function (d1) {
             if ($$.hiddenTargetIds.indexOf(d1.id) >= 0) { return "M 0 0"; }
 
             var d = {