Fixes some tick of categories are missing to display

src/category.js

@@ -1,4 +1,4 @@
 c3_chart_internal_fn.categoryName = function (i) {
-    var config = this.config;
-    return i < config.axis_x_categories.length ? config.axis_x_categories[i] : i;
+    var config = this.config, categoryIndex = Math.ceil(i);
+    return i < config.axis_x_categories.length ? config.axis_x_categories[categoryIndex] : i;
 };