Add support for legend woth empty series

src/data.convert.js

@@ -86,6 +86,13 @@ c3_chart_internal_fn.convertColumnsToData = function (columns) {
             new_rows[j - 1][key] = columns[i][j];
         }
     }
+    if(!new_rows.length){
+        new_rows.empty = true;
+        new_rows[0] = {};
+        for (i = 0; i < columns.length; i++) {
+            new_rows[0][columns[i][0]] = undefined;
+        }
+    }
     return new_rows;
 };
 c3_chart_internal_fn.convertDataToTargets = function (data, appendXs) {

src/data.js

@@ -137,9 +137,10 @@ c3_chart_internal_fn.getMaxDataCountTarget = function (targets) {
 };
 c3_chart_internal_fn.getEdgeX = function (targets) {
     var $$ = this;
-    return !targets.length ? [0, 0] : [
-        $$.d3.min(targets, function (t) { return t.values[0].x; }),
-        $$.d3.max(targets, function (t) { return t.values[t.values.length - 1].x; })
+    var filtered = targets.filter(function(t){ return !!t.values.length; });
+    return !filtered.length ? [0, 0] : [
+        $$.d3.min(filtered, function (t) { return t.values[0].x; }),
+        $$.d3.max(filtered, function (t) { return t.values[t.values.length - 1].x; })
     ];
 };
 c3_chart_internal_fn.mapToIds = function (targets) {

src/domain.js

@@ -62,7 +62,10 @@ c3_chart_internal_fn.getYDomainMax = function (targets) {
 };
 c3_chart_internal_fn.getYDomain = function (targets, axisId, xDomain) {
     var $$ = this, config = $$.config,
-        targetsByAxisId = targets.filter(function (t) { return $$.getAxisId(t.id) === axisId; }),
+        targetsByAxisId = targets.filter(function (t) { 
+            if($$.getAxisId(t.id) !== axisId) return false;
+            return !!t.values.length;
+        }),
         yTargets = xDomain ? $$.filterByXDomain(targetsByAxisId, xDomain) : targetsByAxisId,
         yMin = axisId === 'y2' ? config.axis_y2_min : config.axis_y_min,
         yMax = axisId === 'y2' ? config.axis_y2_max : config.axis_y_max,