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    <h2>dygraphs Data Format</h2>

    <p>When you create a Dygraph object, your code looks something like
    this:</p>

    <code>
      g = new Dygraph(document.getElementById("div"),
                      <i>data</i>,
                      { <i>options</i> });
    </code>

    <p>This document is about what you can put in the <i>data</i>
    parameter.</p>

    <p>There are five types of input that dygraphs will accept:</p>
    <ol>
      <li><a href="#csv">CSV data</a>
      <li><a href="#url">URL</a>
      <li><a href="#array">array (native format)</a>
      <li><a href="#function">function</a>
      <li><a href="#datatable">DataTable</a>
    </ol>

    <p>These are all discussed below. If you're trying to debug why your input
    won't parse, <b>check the JS error console</b>. dygraphs tries to log
    informative errors explaining what's wrong with your data, and these can
    often point you in the right direction.</p>

    <p>There are several options which affect how your input data is
    interpreted. These are:
    <ul>
      <li> <i>xValueParser</i> affects CSV only.
      <li> <i>errorBars</i> affects all input types.
      <li> <i>customBars</i> affects all input types.
      <li> <i>fractions</i> affects all input types.
      <li> <i>labels</i> affects all input types.
    </ul>
    </p>

    <a name="csv"><h3>CSV</h3>
    <p>Here's an example of what CSV data should look like:</p>
    <pre>
    Date,Series1,Series2
    2009/07/12,100,200  # comments are OK on data lines
    2009/07/19,150,201
    </pre>

    <p>"CSV" is actually a bit of a misnomer: the data can be tab-delimited,
    too. The delimiter is set by the <i>delimiter</i> option. It default to ",".
    If no delimiter is found in the first row, it switches over to tab.</p>

    <p>CSV parsing can be split into three parts: headers, x-value and
    y-values.</p>

    <h4>Headers</h4>
    <p>If you don't specify the <i>labels</i> option, dygraphs will look at the
    first line of your CSV data to get the labels. If you see numbers for series
    labels when you hover over the dygraph, it's likely because your first line
    contains data but is being parsed as a label. The solution is to either add
    a header line or specify the labels like this:</p>

    <code>
      new Dygraph(el,
                  "2009/07/12,100,200\n" +
                  "2009/07/19,150,201\n",
                  { labels: [ "Date", "Series1", "Series2" ] });
    </code>

    <h4>x-values</h4>
    <p>Once the headers are parsed, dygraphs needs to determine what the type of
    the x values is. They're either dates or numbers. To make this
    determination, it looks at the first column of the first row ("2009/07/12"
    in the example above). Here's the heuristic: if it contains a '-' or a '/',
    or otherwise doesn't parse as a float, the it's a date. Otherwise, it's a
    number.</p>

    <p>Once the type is determined, that doesn't mean all the values will parse
    correctly. The general rule is:<p>

    <ul>
      <li>For dates, your strings have to be parseable by <i>Date.parse</i>.
      <li>For numbers, your strings have to be parseable by <i>parseFloat</i>.
    </ul>

    <p>You can manually verify this using a JavaScript console. If a value
    doesn't parse, dygraphs will put a warning about it on your console. But
    beware: different browsers support different date formats!</p>

    <p>Here are some valid date formats:</p>
    <ul>
      <li>2009-07-12</li>
      <li>2009/07/12</li>
      <li>2009/07/12 12</li>
      <li>2009/07/12 12:34</li>
      <li>2009/07/12 12:34:56</li>
    </ul>

    <p>If you specify the <i>xValueParser</i> option, then all this detection is
    bypassed and your function is called instead. Your parser function takes in
    a string and needs to return a number. For dates/times, you should return
    milliseconds since epoch. You may also want to specify a few other options
    to make sure that everything gets displayed properly.<p>

    <p>Here's code which parses a CSV file with unix timestamps in the first
    column:</p>

    <code>
      new Dygraph(el,
                  "Date,Series1,Series2\n" +
                  "1247382000,100,200\n" +
                  "1247986800,150,201\n",
                  {
                    xValueFormatter: Dygraph.dateString_,
                    xValueParser: function(x) { return 1000*parseInt(x); },
                    xTicker: Dygraph.dateTicker
                  });
    </code>

    <h4>y-values</h4>
    <p>Dependent (y-axis) values are simpler than x-values because they're
    always numbers. The complexity here comes from the various ways that you can
    specify the uncertainty in your measurements.<p>

    <p>If your y-values are just numbers, then they need to be parseable by
    JavaScript's parseFloat function. Acceptable formats include:</p>

    <ul>
      <li>12
      <li>-12
      <li>12.
      <li>12.3
      <li>1.24e+1
      <li>-1.24e+1
    </ul>

    <p>If you have missing data, just leave the column blank (your CSV file will
    probably contain a ",," in it).</p>

    <p>If your numbers have uncertainty associated with them, then there are
    three basic ways to express this: using fractions, standard deviations or
    explicit ranges.</p>

    <h5>Fractions</h5>
    <p>If you specify the <i>fractions</i> option, then your data will all be
    interpreted as ratios between zero and one. This is often the case if you're
    plotting a percentage.</p>

    <code>
      new Dygraph(el,
                  "X,Frac1,Frac2\n" +
                  "1,1/2,3/4\n"+
                  "2,1/3,2/3\n"+
                  "3,2/3,17/49\n"+
                  "4,25/30,100/200",
                  { fractions: true });
    </code>

    <p>Why not just divide the fractions out yourself? There are two attractive
    reasons not to:</p>

    <ul>
      <li>If you set both <i>fractions</i> and <i>errorBars</i>, then the
    denominator is interpreted as a sample size and dygraphs will plot <a
      href="http://en.wikipedia.org/wiki/Binomial_proportion_confidence_interval">Wilson
      binomial proportion confidence intervals</a> around each point.

      <li>If you set <i>showRoller</i>, then dygraphs will combine the values as
      fractions. If two point are <i>a/b</i> and <i>c/d</i>, it will plot
      <i>(a+b) / (c+d)</i> rather than <i>(a/b + c/d) / 2</i>, which is what
      you'd get if you divided the fractions through. This will also shrink the
      confidence intervals.</li>
    </ul>

    <h5>Standard Deviations</h5>
    <p>Often you have a measurement and also a measure of its uncertainty: a
    standard deviation. If you specify the <i>errorBars</i> option, dygraphs
    will look for alternating value and standard deviation columns in your CSV
    data.  Here's what it should look like:</p>

    <code>
      new Dygraph(el,
                  "X,Y1,Y2\n" +
                  "1,10,5,20,5\n" +
                  "2,12,5,22,5\n",
                  { errorBars: true });
    </code>

    <p>The "5" values are standard deviations. When each point is plotted, a
    2-standard deviation region around it is shaded, resulting in a 95%
    confidence interval. If you want more or less confidence, you can set the
    <i>sigma</i> option to something other than 2.0.</p>

    <p>When you roll data with standard deviations, dygraphs will plot the
    average of your values in each rolling period and the RMS value of your
    standard deviations: sqrt(std1 + std2 + std3 + ... + stdN)/N.</p>

    <h5>Custom error bars</h5>
    <p>Sometimes your data has asymetric uncertainty or you want to specify
    something else with the error bars around a point. One example of this is
    the "temperatures" demo on the <a href="http://danvk.org/dygraphs">dygraphs
      home page.</a>, where the point is the daily average and the bars denote
    the low and high temperatures for the day.</p>

    <p>To specify this format, set the <i>customBars</i> option. Your CSV values
    should each be three numbers separated by semicolons ("low;mid;high").
    Here's an example:</p>

    <code>
      new Dygraph(el,
                  "X,Y1,Y2\n" +
                  "1,10;20;30,20;5;25\n" +
                  "2,10;25;35,20;10;25\n",
                  { customBars: true });
    </code>

    <p>The middle value need not lie between the low and high values. If you set
    a rolling period, the three values will all be averaged independently.</p>


    <a name="url"><h3>URL</h3>
    <p>If you pass in a URL, dygraphs will issue an XMLHttpRequest for it and
    attempt to parse the returned data as CSV.
    </p>

    <p><i>Common problems</i>. Make sure the URL is accessible and returns data
    in text format (as opposed to a CSV file with an HTML header). You can see
    what the response looks like by checking your JS console or by requesting
    the URL yourself.</p>


    <a name="array"><h3>Array (native format)</h3>
    <p>If you'll be constructing your data set from a server-side program (or
    from JavaScript) then you're better off producing an array than CSV data.
    This saves the cost of parsing the CSV data and also avoids common parser
    errors.</p>

    <p>The downside is that it's harder to look at your data (you'll need to use
    a JS debugger) and that the data format is a bit less clear for values with
    uncertainties.</p>

    <p>Here's an example of "native format":</p>

    <code>
      new Dygraph(document.getElementById("graphdiv2"),
                  [
                    [1,10,100],
                    [2,20,80],
                    [3,50,60],
                    [4,70,80]
                  ],
                  {
                    labels: [ "x", "A", "B" ]
                  });
    </code>

    <h4>Headers</h4>
    <p>Headers for native format must be specified via the <i>labels</i>
    option. There's no other way to set them.</p>

    <h4>x-values</h4>
    <p>If you want your x-values to be dates, you'll need to use specify a Date
    object in the first column. Otherwise, specify a number. Here's a sample
    array with dates on the x-axis:</p>

    <code>
      [
        [ new Date("2009/07/12"), 100, 200 ],
        [ new Date("2009/07/19"), 150, 220 ]
      ]
    </code>

    <h4>y-values</h4>
    <p>You can specify <i>errorBars</i>, <i>fractions</i> or <i>customBars</i>
    with the array format. If you specify any of these, the values become arrays
    (rather than numbers). Here's what the format looks like for each one:</p>

    <code>
      <i>errorBars</i>: [x, [value1, std1], [value2, std2], ...]
      <i>fractions</i>: [x, [num1, den1], [num2, den2], ...]
      <i>customBars</i>: [x, [low1, val1, high1], [low2, val2, high2], ...]
    </code>

    <p>To specify missing data, set the value to null. You may not set a value
    inside an array to null. Use null instead of the entire array.</p>

    <a name="function"><h3>Functions</h3>

    <p>You can specify a function that returns any of the other types. If
    <i>x</i> is a valid piece of dygraphs input, then so is</p>

    <code>
      function() { return x; }
    </code>

    Functions can return strings, arrays, data tables, URLs, or any other data type.

    <a name="datatable"><h3>DataTable</h3>
    <p>You can also specify a Google Visualization Library <a
      href="http://code.google.com/apis/visualization/documentation/reference.html#DataTable">DataTable</a>
    object as your input data. This lets you easily switch between dygraphs and
    other gviz visualizations such as the Annotated Timeline. It also lets you
    embed a Dygraph in a Google Spreadsheet.</p>

    <p>You'll need to set your first column's type to one of "number", "date"
    or "datetime".</p>

    <pre>
    DataTable TODO:
    - When to use Dygraph.GvizWrapper
    - how to specify fractions
    - how to specify missing data
    - how to specify value + std. dev.
    - how to specify [low, middle, high]
    - walkthrough of embedding a gadget in google docs/on a web page
    - walkthrough of using std. dev. in a spreadsheet chart
    </pre>

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