Fix error about event rect widt when no ticks

c3.js

@@ -6918,7 +6918,7 @@
                 length = axis.g.select('path.domain').node().getTotalLength() - outerTickSize * 2;
                 interval = length / axis.g.selectAll('line').size();
             }
-            return interval;
+            return interval === Infinity ? 0 : interval;
         };
         axis.ticks = function () {
             if (!arguments.length) { return tickArguments; }

src/c3.axis.js

@@ -270,7 +270,7 @@ function c3_axis(d3, params) {
             length = axis.g.select('path.domain').node().getTotalLength() - outerTickSize * 2;
             interval = length / axis.g.selectAll('line').size();
         }
-        return interval;
+        return interval === Infinity ? 0 : interval;
     };
     axis.ticks = function () {
         if (!arguments.length) { return tickArguments; }