remove 'kernel' from violin attributes

- as non-gaussian kernels may require us to make soft spanmode bounds
  kernel-dependent, which will require some trial-and-error
- as most other libraries don't support non-guassian kernel, let's
  defer this.

Author: etpinard

src/traces/violin/attributes.js

@@ -25,16 +25,6 @@ module.exports = {
         ].join(' ')
     }),
 
-    kernel: {
-        valType: 'enumerated',
-        values: ['gaussian', 'epanechnikov'],
-        dflt: 'gaussian',
-        role: 'info',
-        editType: 'calc',
-        description: [
-            'Determines which kernel is used to compute the kernel density estimation.'
-        ].join(' ')
-    },
     bandwidth: {
         valType: 'number',
         min: 0,

src/traces/violin/defaults.js

@@ -25,7 +25,6 @@ module.exports = function supplyDefaults(traceIn, traceOut, defaultColor, layout
     boxDefaults.handleSampleDefaults(traceIn, traceOut, coerce, layout);
     if(traceOut.visible === false) return;
 
-    coerce('kernel');
     coerce('bandwidth');
     coerce('scalegroup', traceOut.name);
     coerce('scalemode');

src/traces/violin/helpers.js

@@ -10,18 +10,18 @@
 
 var Lib = require('../../lib');
 
+// Maybe add kernels more down the road,
+// but note that the default `spanmode: 'soft'` bounds might have
+// to become kernel-dependent
 var kernels = {
     gaussian: function(v) {
         return (1 / Math.sqrt(2 * Math.PI)) * Math.exp(-0.5 * v * v);
-    },
-    epanechnikov: function(v) {
-        return Math.abs(v) <= 1 ? 0.75 * (1 - v * v) : 0;
     }
 };
 
 exports.makeKDE = function(calcItem, trace, vals) {
     var len = vals.length;
-    var kernel = kernels[trace.kernel];
+    var kernel = kernels.gaussian;
     var bandwidth = calcItem.bandwidth;
     var factor = 1 / (len * bandwidth);