How to change line color in d3js according to axis value?

Question

I want to change color for this line chart if X > 100 I want it to turn \"red\"

Is there a way I can use condition within stroke color style based on value of X ?

Answer 1

Here's a quick example that I've come up with:

Instead of a single path, let's use multiple lines.

We'll need to convert our data to have the following properties:

[
  {
    x1: currentX,
    y1: currentY,
    x2: nextX,
    y2: nextY
  },
  ...
]

Then we can draw them with a conditional stroke attribute based on data:

var lines = svgContainer.selectAll('line')
    .data(lineData)
    .enter()
    .append('line')
    .attr('x1', function(d) { return d.x1; })
    .attr('y1', function(d) { return d.y1; })
    .attr('x2', function(d) { return d.x2; })
    .attr('y2', function(d) { return d.y2; })
    .attr("stroke", function (d) {
        return (d.x > 50) ? 'red' : 'blue';
    })
    .attr("fill", "none")
    .attr("stroke-width", 2);

Here's a demo:

var lineData = [
        {"x": 1, "y": 5},
        {"x": 20, "y": 20},
                  { "x": 40,  "y": 10}, { "x": 60,  "y": 40},
                  { "x": 80,  "y": 5},  { "x": 100, "y": 60},
                { "x": 120,  "y": 15},  { "x": 140, "y": 40},
                { "x": 160,  "y": 25},  { "x": 180, "y": 20},
                { "x": 200,  "y": 15},  { "x": 220, "y": 80},
                { "x": 240,  "y": 35},  { "x": 260, "y": 60}
               ];
 
 //This is the accessor function we talked about above
var lineFunction = d3.svg.line()
                          .x(function(d) { return d.x; })
                          .y(function(d) { return d.y; })
                         .interpolate("linear");

//The SVG Container
var svgContainer = d3.select("body").append("svg")
                                    .attr("width", 200)
                                    .attr("height", 200);

lineData = lineData.map(function (point, index, arr) {
    var next = arr[index + 1],
        prev = arr[index - 1];
    return {
        x: point.x,
        y: point.y,
        x1: point.x,
        y1: point.y,
        x2: (next) ? next.x : prev.x,
        y2: (next) ? next.y : prev.y
    };
});

var lines = svgContainer.selectAll('line')
        .data(lineData)
        .enter()
        .append('line')
        .attr('x1', function(d) { return d.x1; })
        .attr('y1', function(d) { return d.y1; })
        .attr('x2', function(d) { return d.x2; })
        .attr('y2', function(d) { return d.y2; })
        .attr("stroke", function (d) {
            return (d.x > 50) ? 'red' : 'blue';
        })
        .attr("fill", "none")
        .attr("stroke-width", 2);