Fix isInterpolationType to accept type

c3.js

@@ -1251,6 +1251,8 @@
             donut_title: "",
             donut_expand: {},
             donut_expand_duration: 50,
+            // spline
+            spline_interpolation_type: 'cardinal',
             // region - region to change style
             regions: [],
             // tooltip - show when mouseover on each data
@@ -2819,8 +2821,9 @@
 
 
     c3_chart_internal_fn.getInterpolate = function (d) {
-        var $$ = this;
-        return $$.isSplineType(d) ? "cardinal" : $$.isStepType(d) ? $$.config.line_step_type : "linear";
+        var $$ = this,
+            interpolation = $$.isInterpolationType($$.config.spline_interpolation_type) ? $$.config.spline_interpolation_type : 'cardinal';
+        return $$.isSplineType(d) ? interpolation : $$.isStepType(d) ? $$.config.line_step_type : "linear";
     };
 
     c3_chart_internal_fn.initLine = function () {
@@ -3548,6 +3551,9 @@
     c3_chart_internal_fn.barOrLineData = function (d) {
         return this.isBarType(d) || this.isLineType(d) ? d.values : [];
     };
+    c3_chart_internal_fn.isInterpolationType = function (type) {
+        return ['linear', 'linear-closed', 'basis', 'basis-open', 'basis-closed', 'bundle', 'cardinal', 'cardinal-open', 'cardinal-closed', 'monotone'].indexOf(type) >= 0;
+    };
 
     c3_chart_internal_fn.initGrid = function () {
         var $$ = this, config = $$.config, d3 = $$.d3;

src/shape.js

@@ -66,7 +66,7 @@ c3_chart_internal_fn.isWithinShape = function (that, d) {
 
 
 c3_chart_internal_fn.getInterpolate = function (d) {
-    var $$ = this;
-    var interpolation = $$.isInterpolationType() ? $$.config.spline_interpolation_type : 'cardinal';
+    var $$ = this,
+        interpolation = $$.isInterpolationType($$.config.spline_interpolation_type) ? $$.config.spline_interpolation_type : 'cardinal';
     return $$.isSplineType(d) ? interpolation : $$.isStepType(d) ? $$.config.line_step_type : "linear";
 };

src/type.js

@@ -89,6 +89,6 @@ c3_chart_internal_fn.lineOrScatterData = function (d) {
 c3_chart_internal_fn.barOrLineData = function (d) {
     return this.isBarType(d) || this.isLineType(d) ? d.values : [];
 };
-c3_chart_internal_fn.isInterpolationType = function () {
-    return ['linear', 'linear-closed', 'basis', 'basis-open', 'basis-closed', 'bundle', 'cardinal', 'cardinal-open', 'cardinal-closed', 'monotone'].indexOf(this.config.spline_interpolation_type) >= 0;
+c3_chart_internal_fn.isInterpolationType = function (type) {
+    return ['linear', 'linear-closed', 'basis', 'basis-open', 'basis-closed', 'bundle', 'cardinal', 'cardinal-open', 'cardinal-closed', 'monotone'].indexOf(type) >= 0;
 };