Being a newcomer to D3 and JavaScript, I'm hoping someone can help me clarify this simple point.
I am creating a scatter graph with draggable points using code that closely resembles the solution provided in this Stack Overflow question. When I construct a dataset as an array of [x, y] pairs and reference them as d[0] and d[1], everything functions as expected. However, if I use an array of objects with attributes x and y, referring to them as d.x and d.y, the plot looks the same but the dragging behavior does not work properly – when a point is clicked, it moves below the x-axis.
The following code performs correctly:
<!DOCTYPE html>
<svg width="500" height="350"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>
let svg = d3.select("svg"),
margin = {top: 20, right: 20, bottom: 30, left: 50},
width = +svg.attr("width") - margin.left - margin.right,
height = +svg.attr("height") - margin.top - margin.bottom;
//Create fake data as an array of [x, y] pairs
let m=3.0, c=15.0;
let points = d3.range(1, 10).map(function(i) {
let x=i * width / 10;
let noise=Math.random()*500;
let y=m*x+c + noise;
return [x, y];
});
let x = d3.scaleLinear()
.rangeRound([0, width]);
let y = d3.scaleLinear()
.rangeRound([height, 0]);
let xAxis = d3.axisBottom(x),
yAxis = d3.axisLeft(y);
let line = d3.line()
.x(function(d) { return x(d[0]); })
.y(function(d) { return y(d[1]); });
let drag = d3.drag()
.on('start', dragstarted)
.on('drag', dragged)
.on('end', dragended);
svg.append('rect')
.attr('class', 'zoom')
.attr('cursor', 'move')
.attr('fill', 'none')
.attr('pointer-events', 'all')
.attr('width', width)
.attr('height', height)
.attr('transform', 'translate(' + margin.left + ',' + margin.top + ')')
let focus = svg.append("g")
.attr("transform", "translate(" + margin.left + "," + margin.top + ")");
x.domain(d3.extent(points, function(d) { return d[0]; }));
y.domain(d3.extent(points, function(d) { return d[1]; }));
focus.append("path")
.datum(points)
.attr("fill", "none")
.attr("stroke", "steelblue")
.attr("stroke-linejoin", "round")
.attr("stroke-linecap", "round")
.attr("stroke-width", 1.5)
.attr("d", line);
focus.selectAll('circle')
.data(points)
.enter()
.append('circle')
.attr('r', 5.0)
.attr('cx', function(d) { return x(d[0]); })
.attr('cy', function(d) { return y(d[1]); })
.style('cursor', 'pointer')
.style('fill', 'steelblue');
focus.selectAll('circle')
.call(drag);
focus.append('g')
.attr('class', 'axis axis--x')
.attr('transform', 'translate(0,' + height + ')')
.call(xAxis);
focus.append('g')
.attr('class', 'axis axis--y')
.call(yAxis);
function dragstarted(d) {
d3.select(this).raise().classed('active', true);
}
function dragged(d) {
d[0] = x.invert(d3.event.x);
d[1] = y.invert(d3.event.y);
d3.select(this)
.attr('cx', x(d[0]))
.attr('cy', y(d[1]))
focus.select('path').attr('d', line);
}
function dragended(d) {
d3.select(this).classed('active', false);
}
</script>However, this code exhibits strange behavior when dragging the points:
<!DOCTYPE html>
<svg width="500" height="350"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>
let svg = d3.select("svg"),
margin = {top: 20, right: 20, bottom: 30, left: 50},
width = +svg.attr("width") - margin.left - margin.right,
height = +svg.attr("height") - margin.top - margin.bottom;
//Generate data as an array of objects, each with attributes x and y
let m=3.0, c=15.0;
let points = d3.range(1, 10).map(function(i) {
let x_val=i * width / 10;
let noise=Math.random()*500;
let y_val=m*x_val+c + noise;
return {
x:x_val,
y:y_val
};
});
let x = d3.scaleLinear()
.rangeRound([0, width]);
let y = d3.scaleLinear()
.rangeRound([height, 0]);
let xAxis = d3.axisBottom(x),
yAxis = d3.axisLeft(y);
let line = d3.line()
.x(function(d) { return x(d.x); })
.y(function(d) { return y(d.y); });
let drag = d3.drag()
.on('start', dragstarted)
.on('drag', dragged)
.on('end', dragended);
svg.append('rect')
.attr('class', 'zoom')
.attr('cursor', 'move')
.attr('fill', 'none')
.attr('pointer-events', 'all')
.attr('width', width)
.attr('height', height)
.attr('transform', 'translate(' + margin.left + ',' + margin.top + ')');
let focus = svg.append("g")
.attr("transform", "translate(" + margin.left + "," + margin.top + ")");
x.domain(d3.extent(points, function(d) { return d.x; }));
y.domain(d3.extent(points, function(d) { return d.y; }));
focus.append("path")
.datum(points)
.attr("fill", "none")
.attr("stroke", "steelblue")
.attr("stroke-linejoin", "round")
.attr("stroke-linecap", "round")
.attr("stroke-width", 1.5)
.attr("d", line);
focus.selectAll('circle')
.data(points)
.enter()
.append('circle')
.attr('r', 5.0)
.attr('cx', function(d) { return x(d.x); })
.attr('cy', function(d) { return y(d.y); })
.style('cursor', 'pointer')
.style('fill', 'steelblue');
focus.selectAll('circle')
.call(drag);
focus.append('g')
.attr('class', 'axis axis--x')
.attr('transform', 'translate(0,' + height + ')')
.call(xAxis);
focus.append('g')
.attr('class', 'axis axis--y')
.call(yAxis);
function dragstarted(d) {
d3.select(this).raise().classed('active', true);
}
function dragged(d) {
d.x = x.invert(d3.event.x);
d.y = y.invert(d3.event.y);
d3.select(this)
.attr('cx', x(d.x))
.attr('cy', y(d.y));
focus.select('path').attr('d', line);
}
function dragended(d) {
d3.select(this).classed('active', false);
}
</script>If anyone could shed light on why these two versions produce different outcomes, despite only changing the data structure, I would appreciate your explanation! Thank you!