Chapter 02Binding Object Data

We are not limited to binding primitive type values to the elements in a visualization. We can also bind objects (and arrays). Take for example, the following array containing estimates for the homeless population in five U.S. cities in January of 2018 provided by the United States Interagency Council on Homelessness.

The data consists of an array of object where each object has a state and a population property.

var homeless = [
    {state: "California", population: 129972 },
    {state: "New York", population: 91897 },
    {state: "Florida", population: 31030 },
    {state: "Texas", population: 25310 },
    {state: "Washington", population: 22304 }];

We can bind this data to an empty selection and create text and rect elements for each object using join. Once the visual elements are added we can use the state and population properties of the bound data to set the text labels and to determine the lengths of the rectangles, respectively.

<script>
  var homeless = [
    {state: "California", population: 129972 },
    {state: "New York", population: 91897 },
    {state: "Florida", population: 31030 },
    {state: "Texas", population: 25310 },
    {state: "Washington", population: 22304 }];
    
  var bar = d3.select("#bargraph1")
    .selectAll()
    .data(homeless);
    
  bar.join("text")
    .text((d) => d.state)
    .attr('x', 96)
    .attr('text-anchor', 'end')
    .attr('y', (d, i) => i * 20 + 17);
    
  bar.join("rect")
    .attr("height", 19)
    .attr("width", (d) => d.population/500)
    .attr("x", 100)
    .attr("y", (d, i) => i * 20)
    .attr("fill", "pink");
</script>

<svg id="bargraph1" width="400" height="100" ></svg>