Fix x axis clip height

c3.js

@@ -4319,7 +4319,7 @@
         return forHorizontal ? $$.width + 2 + left + right : $$.margin.left + 20;
     };
     c3_chart_internal_fn.getAxisClipHeight = function (forHorizontal) {
-        return forHorizontal ? this.margin.bottom : this.height + 8;
+        return (forHorizontal ? this.margin.bottom : this.height) + 8;
     };
     c3_chart_internal_fn.getXAxisClipWidth = function () {
         var $$ = this;

src/clip.js

@@ -37,7 +37,7 @@ c3_chart_internal_fn.getAxisClipWidth = function (forHorizontal) {
     return forHorizontal ? $$.width + 2 + left + right : $$.margin.left + 20;
 };
 c3_chart_internal_fn.getAxisClipHeight = function (forHorizontal) {
-    return forHorizontal ? this.margin.bottom : this.height + 8;
+    return (forHorizontal ? this.margin.bottom : this.height) + 8;
 };
 c3_chart_internal_fn.getXAxisClipWidth = function () {
     var $$ = this;