Lightweight fix for unexpected category behavior when multiple series are used.

Fixes #1643

src/data.convert.js

@@ -146,13 +146,21 @@ c3_chart_internal_fn.convertDataToTargets = function (data, appendXs) {
             id: convertedId,
             id_org: id,
             values: data.map(function (d, i) {
-                var xKey = $$.getXKey(id), rawX = d[xKey], x = $$.generateTargetX(rawX, id, i),
+                var xKey = $$.getXKey(id), rawX = d[xKey],
-                    value = d[id] !== null && !isNaN(d[id]) ? +d[id] : null;
+                    value = d[id] !== null && !isNaN(d[id]) ? +d[id] : null, x;
                 // use x as categories if custom x and categorized
-                if ($$.isCustomX() && $$.isCategorized() && index === 0 && rawX) {
+                if ($$.isCustomX() && $$.isCategorized() && rawX) {
-                    if (i === 0) { config.axis_x_categories = []; }
+                    if (index === 0 && i === 0) {
+                        config.axis_x_categories = [];
+                    }
+                    x = config.axis_x_categories.indexOf(rawX);
+                    if (x === -1) {
+                        x = config.axis_x_categories.length;
                         config.axis_x_categories.push(rawX);
                     }
+                } else {
+                    x  = $$.generateTargetX(rawX, id, i);
+                }
                 // mark as x = undefined if value is undefined and filter to remove after mapped
                 if (isUndefined(d[id]) || $$.data.xs[id].length <= i) {
                     x = undefined;