Extending dc.js to add a “simpleLineChart” chart

Question

edit See here for the non-working example of what I'm trying to do: http://bl.ocks.org/elsherbini/5814788

I am using dc.js to plot data collected from bee hives at my university. I am pushing new data to the graphs on every database change (using the magic of Meteor). When the database is over 5000 records or so, rerendering the lines gets really slow. So I want to use simplify.js to preprocess the lines before rendering. To see what I'm talking about, go to http://datacomb.meteor.com/. The page freezes after a couple of seconds, so be warned.

I have started to extend dc.js with a simpleLineChart, which would inherit from the existing dc.lineChart object/function. Here is what I have so far:

dc.simpleLineChart = function(parent, chartGroup) {
    var _chart = dc.lineChart(),
        _tolerance = 1,
        _highQuality = false,
        _helperDataArray;

    _chart.tolerance = function (_) {
        if (!arguments.length) return _tolerance;
        _tolerance = _;
        return _chart;
    };

    _chart.highQuality = function (_) {
        if (!arguments.length) return _highQuality;
        _highQuality = _;
        return _chart;
    };

    return _chart.anchor(parent, chartGroup);
}

simplify.js takes in an array of data, a tolerance, and a boolean highQuality, and returns a new array with fewer elements based on it's simplification algorithm.

dc.js uses crossfilter.js. dc.js charts are associated with a particular crossfilter dimension and group. Eventually, it uses the data from someGroup().all() as the data to pass to a d3.svg.line(). I can't find where this is happening in the dc.js source, but this is where I need to intervene. I want to find this method, and override it in the dc.simpleLineChart object that I am making.

I was thinking something like

_chart.theMethodINeedToOverride = function(){
    var helperDataArray = theChartGroup().all().map(function(d) { return {
        x: _chart.keyAccessor()(d), 
        y: _chart.valueAccessor()(d)};})

    var simplifiedData = simplify(helperDataArray, _tolerance, _highQuality)

    g.datum(simplifiedData); // I know I'm binding some data at some point
                             // I'm just not sure to what or when
}

Can anyone help me either identify which method I need to override, or even better, show me how to do so?

dc.js source: https://github.com/NickQiZhu/dc.js/blob/master/dc.js

edit:

I think I may have found the function I need to override. The original function is

function createGrouping(stackedCssClass, group) {
    var g = _chart.chartBodyG().select("g." + stackedCssClass);

    if (g.empty())
        g = _chart.chartBodyG().append("g").attr("class", stackedCssClass);

    g.datum(group.all());

    return g;
}

And I have tried to override it like so

function createGrouping(stackedCssClass, group) {
    var g = _chart.chartBodyG().select("g." + stackedCssClass);

    if (g.empty())
        g = _chart.chartBodyG().append("g").attr("class", stackedCssClass);

    var helperDataArray = group().all().map(function(d) { return {
        x: _chart.keyAccessor()(d), 
        y: _chart.valueAccessor()(d)};})

    var simplifiedData = simplify(helperDataArray, _tolerance, _highQuality)

    g.datum(simplifiedData);

    return g;
}

However, when I make a simpleLineChart, it is just a linechart with a tolerance() and highQuality() method. See here: http://bl.ocks.org/elsherbini/5814788

Answer 1

Well, I pretty much did what I set out to do. http://bl.ocks.org/elsherbini/5814788

The key was to not only modify the createGrouping function, but also the lineY function in the code. (lineY gets set to tell the d3.svg.line() instance how to set the y value of a given point d)

I changed it to

var lineY = function(d, dataIndex, groupIndex) {
    return _chart.y()(_chart.valueAccessor()(d));
};

The way lineY was written before, it was looking up the y value in an array, rather than using the data bound to the group element. This array had it's data set before i made my changes, so it was still using the old, pre-simplification data.