The path for background arc needs to be supplied with data to get it rendered. This went missing. (#2401)

src/arc.js

@@ -471,8 +471,7 @@ 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");
+        backgroundArc.enter().append("path")
-        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"; }