Control whether tooltip is displayed by value

src/tooltip.js

@@ -38,14 +38,16 @@ c3_chart_internal_fn.getTooltipContent = function (d, defaultTitleFormat, defaul
             text = "<table class='" + CLASS.tooltip + "'>" + (title || title === 0 ? "<tr><th colspan='2'>" + title + "</th></tr>" : "");
         }
 
-        name = nameFormat(d[i].name, d[i].ratio, d[i].id, d[i].index);
         value = valueFormat(d[i].value, d[i].ratio, d[i].id, d[i].index);
-        bgcolor = $$.levelColor ? $$.levelColor(d[i].value) : color(d[i].id);
+        if (value !== undefined) {
+            name = nameFormat(d[i].name, d[i].ratio, d[i].id, d[i].index);
+            bgcolor = $$.levelColor ? $$.levelColor(d[i].value) : color(d[i].id);
 
-        text += "<tr class='" + CLASS.tooltipName + "-" + d[i].id + "'>";
-        text += "<td class='name'><span style='background-color:" + bgcolor + "'></span>" + name + "</td>";
-        text += "<td class='value'>" + value + "</td>";
-        text += "</tr>";
+            text += "<tr class='" + CLASS.tooltipName + "-" + d[i].id + "'>";
+            text += "<td class='name'><span style='background-color:" + bgcolor + "'></span>" + name + "</td>";
+            text += "<td class='value'>" + value + "</td>";
+            text += "</tr>";
+        }
     }
     return text + "</table>";
 };