api: relax transform module requirements

- transform module are now required to have a transfrom OR
  calcTransform method

Author: etpinard

src/plot_api/register.js

@@ -61,8 +61,14 @@ function registerTransformModule(newModule) {
 
     var prefix = 'Transform module ' + newModule.name;
 
-    if(typeof newModule.transform !== 'function') {
-        throw new Error(prefix + ' is missing a *transform* function.');
+    var hasTransform = typeof newModule.transform === 'function',
+        hasCalcTransform = typeof newModule.calcTransform === 'function';
+
+
+    if(!hasTransform && !hasCalcTransform) {
+        throw new Error(prefix + ' is missing a *transform* or *calcTransform* method.');
+    }
+
     }
     if(!Lib.isPlainObject(newModule.attributes)) {
         Lib.log(prefix + ' registered without an *attributes* object.');

src/plots/plots.js

@@ -792,7 +792,7 @@ function applyTransforms(fullTrace, fullData, layout) {
         var transform = container[i],
             _module = transformsRegistry[transform.type];
 
-        if(_module) {
+        if(_module && _module.transform) {
             dataOut = _module.transform(dataOut, {
                 transform: transform,
                 fullTrace: fullTrace,

src/transforms/filter.js

@@ -109,10 +109,6 @@ exports.supplyDefaults = function(transformIn) {
     return transformOut;
 };
 
-exports.transform = function(data) {
-    return data;
-};
-
 exports.calcTransform = function(gd, trace, opts) {
     var filtersrc = opts.filtersrc;