Fix conflict

c3.js

@@ -1239,6 +1239,12 @@
         if (yTargets.length === 0) { // use current domain if target of axisId is none
             return axisId === 'y2' ? $$.y2.domain() : $$.y.domain();
         }
+        if (isNaN(yDomainMin)) { // set minimum to zero when not number
+            yDomainMin = 0;
+        }
+        if (isNaN(yDomainMax)) { // set maximum to have same value as yDomainMin
+            yDomainMax = yDomainMin;
+        }
         if (yDomainMin === yDomainMax) {
             yDomainMin < 0 ? yDomainMax = 0 : yDomainMin = 0;
         }

src/domain.js

@@ -76,6 +76,12 @@ c3_chart_internal_fn.getYDomain = function (targets, axisId) {
     if (yTargets.length === 0) { // use current domain if target of axisId is none
         return axisId === 'y2' ? $$.y2.domain() : $$.y.domain();
     }
+    if (isNaN(yDomainMin)) { // set minimum to zero when not number
+        yDomainMin = 0;
+    }
+    if (isNaN(yDomainMax)) { // set maximum to have same value as yDomainMin
+        yDomainMax = yDomainMin;
+    }
     if (yDomainMin === yDomainMax) {
         yDomainMin < 0 ? yDomainMax = 0 : yDomainMin = 0;
     }