This YUI2 DataTable http://developer.yahoo.com/yui/datatable/ extension provides 
a table footer that can display various summary statistics from the data in the 
table.  It is dynamic, so responds to inline data changes or changes to the
display of the table.

The extension to DataTable is in YAHOO_DataTable_colStats.js; the files
YAHOO_DataSource_patch.js and YAHOO_util_Stats.js are both required by it.

To add some column statistics, you'll need:
 1. To make sure your data source has "number" parsers for the relevant fields
 2. To add columnStats: true for the columns that you want summaries for
    in the column definition you pass to the DataTable constructor
 3. To add a config value to your datatable constructor, minimally:

      columnStats: {on: true, stats: ['sum']}

    and maximally:

      columnStats: {on: true, 
        pagedTotals: false, 
        stats: [ {label: 'Grand Total', fn: 'sum'},
           {label: 'Average', fn: 'mean'},
           'median',
           {label: 'Weighted average': 
            fn: function(oStat, oAllStats, oCol) {
              // a custom function, which receives arguments:
              //   - YAHOO.util.Stats instance for the current column,
              //   - An object literal containing all the Stats instances for 
              //       'summarisable' columns
              //   - The column object that we're on
              // and will be called in the scope of the DataTable
              //
              
              // This gives use an array of records to work with, if we need it:
              var rs = this.colStatsGetRecordSet();
              
              // The function should return a number
              return 2;
           }}
        ]}
        
The pagedTotals value in the columnStats config object determines whether the 
summary should be of the entire dataset, or just the visible records (if you've
got a paginator).  pagedTotals === true means that the statistics will be 
summary values for the current page i.e. records currently visible.

A table foot will be added, with one row for each statistic.  Column formatters
will be used if specified in the Column definitions.  The table cells (td 
elements) in the footer will have class "yui-dt-colstat"

The summary stats will update as appropriate when:
 - columns are moved
 - data is sorted
 - columns are removed/added
 - columns are shown/hidden
 - rows are added or deleted
 - data is edited
 - paginator changes
 
There are two public methods added to the datatable:
 colStatsRefresh()        // update and redraw the table footer with the stats
 colStatsGetRecordSet()   // return the recordset to use for calculations.
 
The former is useful if you change settings on the datatable programatically but
datatable doesn't fire useful events to respond to automatically.

The latter is useful if you're passing in your own custom summary function (as
in the 'maximal' example above).  If you are using pagedTotals then this will
just return 'visibile' records, otherwise it'll return them all.




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YAHOO_DataSource_patch.js adds a parseField(key, value) method which provides 
relatively easy access to any parsers specified in the DataSource definition.
This is useful because DataTable does not parse data added (for example by cell
editors), so adding a value 5.2 in fact adds the string value "5.2" to the 
recordset.

YAHOO_util_Stats.js provides a wrapper for an array of numbers and calculates 
various summary statistics from them (on demand).  Most of these values are cached.
This is what does the maths for the DataTable extension.

You could also use this standalone, like this:

  var s = new YAHOO.util.Stats(1,5,3,7,4); // or new YAHOO.util.Stats([1,5,3,7,4]);
  s.addDataValue(4);
  s.sum();   // Sums values
  s.mean();  // Think these are all pretty self-explanatory
  s.median();
  s.min();
  s.max();
  s.removeDataValue(4);  // Removes one of the 4 data points: true if successful
  s.range();
  s.stdev();
  s.variance();
  s.varianceUnbiased();