Improve comment for aggregate and revise some docs.

build/vega-lite-schema.json

@@ -4907,7 +4907,7 @@
       "properties": {
         "aggregate": {
           "$ref": "#/definitions/AggregateOp",
-          "description": "The aggregation operations to apply to the fields, such as sum, average or count. See the aggregate operation reference for more."
+          "description": "The aggregation operations to apply to the fields, such as sum, average or count.\nSee the [full list of supported aggregation operations](https://vega.github.io/vega-lite/docs/aggregate.html#supported-aggregation-operations)\nfor more information."
         },
         "as": {
           "description": "The output field names to use for each aggregated field.",
@@ -4936,7 +4936,7 @@
           "type": "array"
         },
         "summarize": {
-          "description": "Array of objects that contains aggregate information for the transform.",
+          "description": "Array of objects that define aggregate fields.",
           "items": {
             "$ref": "#/definitions/Summarize"
           },

site/docs/legend.md

@@ -46,7 +46,7 @@ Besides `legend` property of each encoding channel, the configuration object ([`
 
 ## Legend Properties
 
-To customize legend, a `legend` object in [an encoding channel's definition](encoding.html) can contain the following groups of properties:
+To customize legends, a `legend` object in [an encoding channel's definition](encoding.html) can contain the following groups of properties:
 
 
 

site/docs/transform.md

@@ -156,7 +156,7 @@ A DateTime object must have at least one of the following properties:
   ...
   "transform": [
     {
-      "summarize": {"aggregate": ..., "field": ..., "as": ...,
+      "summarize": [{"aggregate": ..., "field": ..., "as": ...}],
       "groupby": [...]
     } // Summarize Transform
      ...

site/usage/compile.md

@@ -20,13 +20,13 @@ If you want access to the compiled Vega spec from a Javascript program, you can
 var vgSpec = vl.compile(vlSpec).spec;
 ```
 
-You could also pass in a customize logger as an optional parameter, which allow you configure your output messages.
+By default, warnings and errors are printed to the JavaScript console. To customize how errors and warnings are handled, you can pass a customize logger to the compile function. 
 
 ```js
 var vgSpec = vl.compile(vlSpec, logger).spec;
 ```
 
-A custom logger implementation should implement the following interface:
+A custom logger should implement the following interface:
 
 ```typescript
 interface LoggerInterface {

src/transform.ts

@@ -66,7 +66,7 @@ export interface TimeUnitTransform {
 
 export interface SummarizeTransform {
   /**
-   * Array of objects that contains aggregate information for the transform.
+   * Array of objects that define aggregate fields.
    */
   summarize: Summarize[];
 
@@ -78,7 +78,9 @@ export interface SummarizeTransform {
 
 export interface Summarize {
   /**
-   * The aggregation operations to apply to the fields, such as sum, average or count. See the aggregate operation reference for more.
+   * The aggregation operations to apply to the fields, such as sum, average or count.
+   * See the [full list of supported aggregation operations](https://vega.github.io/vega-lite/docs/aggregate.html#supported-aggregation-operations)
+   * for more information.
    */
   aggregate: AggregateOp;