Intention Hidden In Implementation And Misty Edge of Validity - why does this function do what it does and under what assumptions does it operate? See http://wondersofcode.wordpress.com/2013/05/05/intention-hidden-in-implementation-and-misty-edge-of-validity/

augmentSeriesWithPreviousValues.js

// compare two series and add the delta to the data for the current period
augmentSeriesWithPreviousValues:function (seriesData, previousSeriesData) {
    var series = seriesData.series;
    seriesData.previous = [];
    var previousSeries = previousSeriesData.series;
    
    ... // checks, computing maxRows, maxColumns, previousCategories
        // that maps category name to series index (id)
    
    for (var seriesId = 0; seriesId < series.length; seriesId++) {
        var category = seriesData.category[seriesId + 1];
        var previousSeriesId = previousCategories[category];
            for (var rowId = 0; rowId < maxRows; rowId++) {
                for (var columnId = 1; columnId < maxColumns; columnId++) {
                    var value = 0;
                    var previousValue = 0;

                    if (previousSeries[previousSeriesId] && previousSeries[previousSeriesId][rowId]) {
                        previousValue = previousSeries[previousSeriesId][rowId][columnId];
                        if (isNaN(previousValue)) {
                            previousValue = 0;
                        }
                    }

                    if (!series[seriesId]) {
                        series[seriesId] = [];
                    }

                    if (!series[seriesId][rowId]) {
                        series[seriesId][rowId] = [];
                    }

                    value = series[seriesId][rowId][columnId];
                    if (isNaN(value)) {
                        value = 0; // <- bug: this value is never used!
                    }

                    if (!seriesData.previous[seriesId]) {
                        seriesData.previous[seriesId] = [];
                    }
                    if (!seriesData.previous[seriesId][rowId]) {
                        seriesData.previous[seriesId][rowId] = [];
                    }

                    seriesData.previous[seriesId][rowId][columnId] = previousValue;
                }
            }
    }
}