fix: backport #1891 to v0.4 (#2421)

src/shape.line.js

@@ -80,7 +80,7 @@ c3_chart_internal_fn.generateDrawLine = function (lineIndices, isSub) {
     if (!config.line_connectNull) { line = line.defined(function (d) { return d.value != null; }); }
     return function (d) {
         var values = config.line_connectNull ? $$.filterRemoveNull(d.values) : d.values,
-            x = isSub ? $$.x : $$.subX, y = yScaleGetter.call($$, d.id), x0 = 0, y0 = 0, path;
+            x = isSub ? $$.subX : $$.x, y = yScaleGetter.call($$, d.id), x0 = 0, y0 = 0, path;
         if ($$.isLineType(d)) {
             if (config.data_regions[d.id]) {
                 path = $$.lineWithRegions(values, x, y, config.data_regions[d.id]);