Animate lines in a line graph with D3.js

Question

I have my line graph, with 2 lines. Data to draw my line graph is pulled from a .csv file.

Can anyone explain how I could start off with an empty graph, and when I click a button, my lines animate across the graph?

Thanks in advance!!

var button=d3.select("#button");

var margin = {top: 30, right: 20, bottom: 50, left: 60},
width = 700 - margin.left - margin.right,
height = 300 - margin.top - margin.bottom;

var parseDate = d3.time.format("%d-%b-%y").parse;


//treats value passed to it as a time/date
//OUTPUT RANGE
var x = d3.time.scale()
    .range([0, width]);

//OUTPUT RANGE  
var y = d3.scale.linear()
    .range([height, 0]);

var xAxis = d3.svg.axis().scale(x)
    .orient("bottom")
    .ticks(5);

var yAxis = d3.svg.axis().scale(y)
    .orient("left")
    .ticks(5);

//assigns coordinates for each piece of data    
var valueline = d3.svg.line()
    .interpolate("linear")
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d.close); });

//second line data
var valueline2 = d3.svg.line()
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d.open); });

//create area for 'area' below line 
var area = d3.svg.area()
    .x(function(d) { return x(d.date); })
    .y0(height)
    .y1(function(d) { return y(d.close); });    

//creates area to draw graph    
var svg = d3.select("body")
    .append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
//groups content    
.append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

function make_x_axis() {
    return d3.svg.axis()
        .scale(x)
        .orient("bottom")
        .ticks(5)
}

function make_y_axis() {
    return d3.svg.axis()
        .scale(y)
        .orient("left")
        .ticks(30)
}

// csv callback function
d3.csv("myData2.csv", function(data) {
    data.forEach(function(d) {
    d.date = parseDate(d.date);
    //+ operator sets any 'close' values to nuneric
    d.close = +d.close;
    d.open = +d.open;   

});

//INPUT DOMAINS
//.extent() returns min and max values of argument
x.domain(d3.extent(data, function(d) { return d.date; }));
//returns max of whichever set of data is bigger
y.domain([0, d3.max(data, function(d) { return Math.max(d.close, d.open); })]);

//draws lines
//passes the valueline array to path object
svg.append("path") // Add the valueline path.
    .attr("class", "line")
    //adds dashed line
    .style("stroke-dasharray", ("5, 9")) // <== This line here!!
    .attr("d", valueline(data));


button.on("click", function() { 
    svg.append("path")      // Add the valueline2 path. 
        .attr("class", "line2")
        .transition()
        .attr("d", valueline2(data));
    })


svg.append("g") // Add the X Axis
    .attr("class", "x axis")
    //moves x axis to bottom of graph
    .attr("transform", "translate(0," + height + ")")
    .call(xAxis);

//text label for x-axis 
svg.append("text") // text label for the x axis
    .attr("transform", "translate(" + (width / 2) + " ," + (height + margin.bottom - 5 ) + ")")
    .style("text-anchor", "middle")
    .text("Date");

svg.append("g") // Add the Y Axis
    .attr("class", "y axis")
    .call(yAxis);

//text label for y-axis 
svg.append("text")
    .attr("transform", "rotate(-90)")
    .attr("y", 0 - margin.left)
    .attr("x",0 - (height / 2))
    //adds extra left padding as original y pos = 0
    .attr("dy", "1em")
    .style("text-anchor", "middle")
    .text("Value");


//adding a title to the graph
svg.append("text")
    .attr("x", (width / 2))
    .attr("y", 0 - (margin.top / 2))
    .attr("text-anchor", "middle")
    .style("font-size", "16px")
    .style("text-decoration", "underline")
    .text("Me Graph Larry");


//draw x axis grid  
svg.append("g")
    .attr("class", "grid")
    .attr("transform", "translate(0," + height + ")")
    .call(make_x_axis()
    .tickSize(-height, 0, 0)
    .tickFormat("")
)

//draw y axis grid
svg.append("g")
    .attr("class", "grid")
    .call(make_y_axis()
    .tickSize(-width, 0, 0)
    .tickFormat("")
)

});<!--d3.csv close-->

Solution

You need to setup a reasonable start value for the animation:

var startvalueline2 = d3.svg.line()
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(0);      })

button.on("click", function() { 
    svg.append("path")                     // Add the valueline2 path. 
        .attr("class", "line2")
        .attr("d", startvalueline2(data)); // set starting position
        .transition()
        .attr("d", valueline2(data));      // set end position
});

You might also have a look at Mike's path transitions page. To see how to implement smooth (non-wobbly) animations when using svg:path.