Question

I’m trying out d3js and I have a problem with getting my first basic column(vertical bar) chart work. The only thing I find a bit difficult to understand is the scaling thing. I want to make the x and y axis ticks with labels but I have the following problems:

First of all here is my data:

{
"regions":
["Federal","Tigray","Afar","Amhara","Oromia","Gambella","Addis Ababa","Dire Dawa","Harar","Benishangul-Gumuz","Somali","SNNPR "],
"institutions":
[0,0,34,421,738,0,218,22,22,109,0,456]
}

1. On the y-axis the values are there but the order is reversed. Here is the code:

   var y = d3.scale.linear().domain([0, d3.max(data.institutions)]).range([0, height]);

then I use this scale to create a y-axis:

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

and add this axis to the svg element

svgContainer.append("g")
        .attr("class", "y axis")
        .call(yAxis)
            .append("text")
            .attr("transform", "rotate(-90)")
            .attr("y", 6)
            .attr("dy", ".71em")
            .style("text-anchor", "end")
            .text("Institutions");

the problem here is that the y-axis start from 0 at the top and with 700 at the bottom which is OK but it should be in reverse order.

The other problem I have it the x-axis. I want to have an ordinal scale since the values I want to put are in the regions names I have above. So here’s what I’ve done.

var x = d3.scale.ordinal()
                    .domain(data.regions.map(function(d) { return d.substring(0, 2); }))
                    .rangeRoundBands([0, width], .1);

then the axis

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

and finally add it to the svg element

svgContainer.append("g")
            .attr("class", "x axis")
            .attr("transform", "translate( 0," + height + ")")
            .call(xAxis);

Here the problem is the ticks as well as the labels appear but they are not spaced out evenly and do not correspond with the center of the rectangles I’m drawing. Here is the complete code so you can see what’s happening.

$(document).ready(function(){

    d3.json("institutions.json", draw);

});

function draw(data){
    var margin = {"top": 10, "right": 10, "bottom": 30, "left": 50}, width = 700, height = 300;

    var x = d3.scale.ordinal()
                    .domain(data.regions.map(function(d) { return d.substring(0, 2); }))
                    .rangeRoundBands([0, width], .1);


    var y = d3.scale.linear()
              .domain([0, d3.max(data.institutions)])
              .range([0, height]);

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

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

    var svgContainer = d3.select("div.container").append("svg")
                           .attr("class", "chart")
                           .attr("width", width + margin.left + margin.right)
                           .attr("height", height + margin.top + margin.bottom)
                          .append("g")
                            .attr("transform", "translate(" +margin.left+ "," +margin.right+ ")");        

    svgContainer.append("g")
            .attr("class", "x axis")
            .attr("transform", "translate( 0," + height + ")")
            .call(xAxis);

    svgContainer.append("g")
                  .attr("class", "y axis")
                  .call(yAxis)
                .append("text")
                  .attr("transform", "rotate(-90)")
                  .attr("y", 6)
                  .attr("dy", ".71em")
                  .style("text-anchor", "end")
                  .text("Institutions");

    svgContainer.selectAll(".bar")
      .data(data.institutions)
      .enter()
        .append("rect")
        .attr("class", "bar")
        .attr("x", function(d, i) {return i* 41;})
        .attr("y", function(d){return height - y(d);})
        .attr("width", x.rangeBand())
        .attr("height", function(d){return y(d);});

}

Answer 1

I put the code to Fiddle: http://jsfiddle.net/GmhCr/4/

Feel free to edit it! I already fixed both problems.

---

To fix the upside-down y-axis just swap the values of the range function arguments:

var y = d3.scale.linear().domain([0, d3.max(data.institutions)]).range([height, 0]);

Do not forget to adjust the code for the bars if you change the scale!

---

The source of the mismatch between bars and the x-axis can be found here:

var x = d3.scale.ordinal()
    .domain(data.regions.map(function(d) {
        return d.substring(0, 2);}))
    .rangeRoundBands([0, width], .1);

svgContainer.selectAll(".bar")
  .data(data.institutions)
  .enter()
    .append("rect")
    .attr("class", "bar")
    .attr("x", function(d, i) {return i* 41;})
    .attr("y", function(d){return height - y(d);})
    .attr("width", x.rangeBand())
    .attr("height", function(d){return y(d);});

You specify the padding for rangeRoundBands at 0.1 but you ignore the padding when computing the x and width values for the bars. This for example is correct with a padding of 0:

var x = d3.scale.ordinal()
    .domain(data.regions.map(function(d) {
        return d.substring(0, 2);}))
    .rangeRoundBands([0, width], 0);

svgContainer.selectAll(".bar").data(data.institutions).enter().append("rect")
    .attr("class", "bar")
    .attr("x", function(d, i) {
        return i * x.rangeBand();
    })
    .attr("y", function(d) {
        return y(d);
    })
    .attr("width", function(){
        return x.rangeBand();
    })
    .attr("height", function(d) {
        return height -y(d);
    });

The padding determines how much of the domain is reserved for padding. When using a width of 700 and a padding of 0.1 exactly 70 pixels are used for padding. This means you have to add 70 / data[“regions”].length pixels to every bar’s x value to make this work with a padding.