Fixes unexpected category behavior when multiple series are used. Fixes #1643 (#1689)

src/data.convert.js

@@ -170,7 +170,7 @@ c3_chart_internal_fn.convertDataToTargets = function (data, appendXs) {
                 var xKey = $$.getXKey(id), rawX = d[xKey],
                     value = d[id] !== null && !isNaN(d[id]) ? +d[id] : null, x;
                 // use x as categories if custom x and categorized
-                if ($$.isCustomX() && $$.isCategorized() && index === 0 && !isUndefined(rawX)) {
+                if ($$.isCustomX() && $$.isCategorized() && !isUndefined(rawX)) {
                     if (index === 0 && i === 0) {
                         config.axis_x_categories = [];
                     }