[#70] add data count widget api

wiki.md

@@ -15,6 +15,7 @@ The entire dc.js library is scoped under **dc** name space. It does not introduc
 * [Bubble Chart [concrete] < Abstract Bubble Chart < CoordinateGrid Chart](#bubble-chart)
 * [Bubble Overlay Chart [concrete] < Abstract Bubble Chart < Base Chart](#bubble-overlay-chart)
 * [Geo Choropleth Chart [concrete] < Single Selection Chart < Color Chart < Base Chart](#geo-choropleth-chart)
+* [Data Count Widget [concrete] < Base Chart](#data-count)
 
 ### Function Chain
 Majority of dc functions are designed to allow function chaining, meaning it will return the current chart instance
@@ -581,7 +582,7 @@ coloring.
 Examples:
 * [Canadian City Crime Stats](http://nickqizhu.github.com/dc.js/crime/index.html)
 
-#### .dc.bubbleOverlay(parent[, chartGroup])
+#### dc.bubbleOverlay(parent[, chartGroup])
 Create a bubble overlay chart instance and attach it to the given parent element.
 
 Parameters:
@@ -623,7 +624,7 @@ chart implementation was inspired by [the great d3 choropleth example](http://bl
 Examples:
 * [US Venture Capital Landscape 2011](http://nickqizhu.github.com/dc.js/vc/index.html)
 
-#### .dc.geoChoroplethChart(parent[, chartGroup])
+#### dc.geoChoroplethChart(parent[, chartGroup])
 Create a choropleth chart instance and attach it to the given parent element.
 
 Parameters:
@@ -663,3 +664,40 @@ chart.overlayGeoJson(statesJson.features, "state", function(d) {
 Set custom geo projection function. Available [d3 geo projection functions](https://github.com/mbostock/d3/wiki/Geo-Projections).
 Default value: albersUsa.
 
+## <a name="data-count" href="#data-count">#</a> Data Count Widget [Concrete] < [Base Chart](#base-chart)
+Data count is a simple widget designed to display total number records in the data set vs. the number records selected
+by the current filters. Once created data count widget will automatically update the text content of the following elements
+under the parent element.
+
+* ".total-count" - total number of records
+* ".filter-count" - number of records matched by the current filters
+
+Examples:
+* [Nasdaq 100 Index](http://nickqizhu.github.com/dc.js/)
+
+#### dc.dataCount(parent[, chartGroup])
+Create a data count widget instance and attach it to the given parent element.
+
+Parameters:
+* parent : string - any valid d3 single selector representing typically a dom block element such as a div.
+* chartGroup : string (optional) - name of the chart group this chart instance should be placed in. Once a chart is placed
+   in a certain chart group then any interaction with such instance will only trigger events and redraw within the same
+   chart group.
+
+Return:
+A newly created data count widget instance
+
+#### .dimension(allData) - **mandatory**
+For data count widget the only valid dimension is the entire data set.
+
+#### .group(groupAll) - **mandatory**
+For data count widget the only valid group is the all group.
+
+```js
+var ndx = crossfilter(data);
+var all = ndx.groupAll();
+
+dc.dataCount(".dc-data-count")
+    .dimension(ndx)
+    .group(all);
+```