Scatterplot Matrix Brushing

The scatterplot matrix visualizations pairwise correlations for multi-dimensional data; each cell in the matrix is a scatterplot. This example uses Anderson's data of iris flowers on the Gaspé Peninsula. Scatterplot matrix design invented by J. A. Hartigan; see also R and GGobi. Data on Iris flowers collected by Edgar Anderson and published by Ronald Fisher.

This version includes a legend created using d3-legend and highlighting on hover, in addition to brushing.

See also this simpler version with brushing and static version without brushing.

<!DOCTYPE html>
<meta charset="utf-8">
<style>

svg {
  font: 10px sans-serif;
  padding: 10px;
}

.axis,
.frame {
  shape-rendering: crispEdges;
}

.axis line {
  stroke: #ddd;
}

.axis path {
  display: none;
}

.cell text {
  font-weight: bold;
  text-transform: capitalize;
}

.frame {
  fill: none;
  stroke: #aaa;
}

circle {
  fill-opacity: .4;
  stroke-width: 0px;
}

circle.hidden {
  fill: #ccc !important;
}
  
circle.hover {
  fill: none;
  fill-opacity: .7;
  stroke-width: 1px;
}
  
.extent {
  fill: #000;
  fill-opacity: .125;
  stroke: #fff;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src = "https://cdnjs.cloudflare.com/ajax/libs/d3-legend/1.10.0/d3-legend.min.js"></script>
<script>

var width = 960,
    size = 200,
    padding = 20;

var x = d3.scale.linear()
    .range([padding / 2, size - padding / 2]);

var y = d3.scale.linear()
    .range([size - padding / 2, padding / 2]);

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom")
    .ticks(6);

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left")
    .ticks(6);

var color = d3.scale.category10();

d3.csv("flowers.csv", function(error, data) {
  if (error) throw error;

traits.forEach(function(trait) {
    domainByTrait[trait] = d3.extent(data, function(d) { return d[trait]; });
  });

xAxis.tickSize(size * n);
  yAxis.tickSize(-size * n);

var brush = d3.svg.brush()
      .x(x)
      .y(y)
      .on("brushstart", brushstart)
      .on("brush", brushmove)
      .on("brushend", brushend);

var svg = d3.select("body").append("svg")
      .attr("width", size * (n + 1) + padding)
      .attr("height", size * n + padding)
    .append("g")
      .attr("transform", "translate(" + padding + "," + padding / 2 + ")");

svg.selectAll(".x.axis")
      .data(traits)
    .enter().append("g")
      .attr("class", "x axis")
      .attr("transform", function(d, i) { return "translate(" + (n - i - 1) * size + ",0)"; })
      .each(function(d) { x.domain(domainByTrait[d]); d3.select(this).call(xAxis); });

svg.selectAll(".y.axis")
      .data(traits)
    .enter().append("g")
      .attr("class", "y axis")
      .attr("transform", function(d, i) { return "translate(0," + i * size + ")"; })
      .each(function(d) { y.domain(domainByTrait[d]); d3.select(this).call(yAxis); });

var cell = svg.selectAll(".cell")
      .data(cross(traits, traits))
    .enter().append("g")
      .attr("class", "cell")
      .attr("transform", function(d) { return "translate(" + (n - d.i - 1) * size + "," + d.j * size + ")"; })
      .each(plot);

// Titles for the diagonal.
  cell.filter(function(d) { return d.i === d.j; }).append("text")
      .attr("x", padding)
      .attr("y", padding)
      .attr("dy", ".71em")
      .text(function(d) { return d.x; });

cell.call(brush);

function plot(p) {
    var cell = d3.select(this);

x.domain(domainByTrait[p.x]);
    y.domain(domainByTrait[p.y]);

cell.append("rect")
        .attr("class", "frame")
        .attr("x", padding / 2)
        .attr("y", padding / 2)
        .attr("width", size - padding)
        .attr("height", size - padding);

cell.selectAll("circle")
        .data(data)
      .enter().append("circle")
    		.attr("class", function(d) { return d.species; })
        .attr("cx", function(d) { return x(d[p.x]); })
        .attr("cy", function(d) { return y(d[p.y]); })
        .attr("r", 4)
        .style("fill", function(d) { return color(d.species); })
    		.style("stroke", function(d) { return color(d.species); });
  }

var brushCell;

// Clear the previously-active brush, if any.
  function brushstart(p) {
    if (brushCell !== this) {
      d3.select(brushCell).call(brush.clear());
      x.domain(domainByTrait[p.x]);
      y.domain(domainByTrait[p.y]);
      brushCell = this;
    }
  }

// Highlight the selected circles.
  function brushmove(p) {
    var e = brush.extent();
    svg.selectAll("circle").classed("hidden", function(d) {
      return e[0][0] > d[p.x] || d[p.x] > e[1][0]
          || e[0][1] > d[p.y] || d[p.y] > e[1][1];
    });
  }

// If the brush is empty, select all circles.
  function brushend() {
    if (brush.empty()) svg.selectAll(".hidden").classed("hidden", false);
  }

d3.select(self.frameElement).style("height", size * n + padding + 20 + "px");
  
  svg.append("g")
    .attr("class", "legendOrdinal")
    .attr("transform", "translate(" + (size * n + 2 * padding) + "," + (size * ((n - 1) / 2) + padding) + ")");

var legendOrdinal = d3.legend.color()
  	.title("Iris Species")  
  	.classPrefix("colorLegend_")
  	.shape("circle")
    .shapePadding(10)
    .scale(color);

svg.select(".legendOrdinal")
    .call(legendOrdinal);
  
  var legendCells = svg.selectAll(".legendOrdinal .colorLegend_cell")
  	.attr('pointer-events', 'all')
  	.on("mouseover", hoveron)
  	.on("mouseout", hoveroff);
  
  legendCells
    .append('rect')
    .attr('class', 'click-capture')
    .style('visibility', 'hidden')
    .attr('x', -10)
    .attr('y', -10)
    .attr('width', 100)
    .attr('height', 20);
  
  // If the brush is empty, select all circles.
  function hoveron(d) {
    svg.selectAll("circle." + d)
      .classed("hover", true)
  }
  
  // If the brush is empty, select all circles.
  function hoveroff(d) {
    svg.selectAll("circle." + d)
      .classed("hover", false)
  }
  
  });

function cross(a, b) {
  var c = [], n = a.length, m = b.length, i, j;
  for (i = -1; ++i < n;) for (j = -1; ++j < m;) c.push({x: a[i], i: i, y: b[j], j: j});
  return c;
}

</script>

https://d3js.org/d3.v3.min.js