D3 V4: How To Make A Path Being Curved When Only Start And End Point Are Known?

I want to use annotations on my horizontal d3 bar chart, which I successfully applied. Currently, an annotation is connected through a to the respective bar using the

Solution 1:

One option I've used is a custom curve with d3 (d3-shape). This allows the code to work with both Canvas and SVG. It also allows linking together any number of points with the prescribed pattern.

The documentation for custom curves is a little useful, but it might be more useful to see an example:

var curve = function(context) {
  var custom = d3.curveLinear(context);
  custom._context = context;
  custom.point = function(x,y) {
    x = +x, y = +y;
    switch (this._point) {
      case 0: this._point = 1; 
        this._line ? this._context.lineTo(x, y) : this._context.moveTo(x, y);
        this.x0 = x; this.y0 = y;        
        break;
      case 1: this._point = 2;
      default: 
        var x1 = this.x0 * 0.5 + x * 0.5;
        var y1 = this.y0 * 0.5 + y * 0.5;
        var m = 1/(y1 - y)/(x1 - x);
        var r = -100; // offset of mid point.
        var k = r / Math.sqrt(1 + (m*m) );
        if (m == Infinity) {
          y1 += r;
        }
        else {
          y1 += k;
          x1 += m*k;
        }     
        this._context.quadraticCurveTo(x1,y1,x,y); 
        this.x0 = x; this.y0 = y;        
        break;
    }
  }
  return custom;
}

Here for the first point we simply record the point, for subsequent points we draw a quadratic curve from the current point ([x,y]) to the previous point ([x0,y0]). In the example above, [x1,y1] is the control point - which is offset a perpendicular amount the line connecting [x,y] and [x0,y0]:

var curve = function(context) {
  var custom = d3.curveLinear(context);
  custom._context = context;
  custom.point = function(x,y) {
    x = +x, y = +y;
    switch (this._point) {
      case 0: this._point = 1; 
        this._line ? this._context.lineTo(x, y) : this._context.moveTo(x, y);
        this.x0 = x; this.y0 = y;        
        break;
      case 1: this._point = 2;
      default: 
        var x1 = this.x0 * 0.5 + x * 0.5;
        var y1 = this.y0 * 0.5 + y * 0.5;
        var m = 1/(y1 - y)/(x1 - x);
        var r = -50; // offset of mid point.
        var k = r / Math.sqrt(1 + (m*m) );
        if (m == Infinity || m == -Infinity) {
          y1 += r;
        }
        else {
          y1 += k;
          x1 += m*k;
        }     
        this._context.quadraticCurveTo(x1,y1,x,y); 
        this.x0 = x; this.y0 = y;        
        break;
    }
  }
  return custom;
}

// Basic horizontal bar graph:
var svg = d3.select("body").append("svg")
  .attr("width",500)
  .attr("height", 400);

var data = [5,6,7];

var x = d3.scaleLinear()
  .domain([0,10])
  .range([0,320]);
  
var line = d3.line()
  .curve(curve)
  .x(function(d) { return d[0]; })
  .y(function(d) { return d[1]; })  
  
var g = svg.selectAll("g")
 .data(data)
 .enter()
 .append("g")
 .attr("transform",function(d,i) {
   return "translate("+[40,i*90+30]+")"
 });
 
g.append("rect")
 .attr("width",x)
 .attr("height", 40)

g.append("path")
  .attr("d", function(d,i) {
     return line([[0,-3],[x(d),-3]])
  })

path {
  stroke-width: 1px;
  stroke: black;
  fill:none;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/5.7.0/d3.min.js"></script>

I'd create a canvas example, but the principle is exactly the same from the line perspective, the only difference is we would use d3.line().context(context).curve(...

Solution 2:

You can draw a <path> with a d attribute, as is described in the SVG path spec (https://www.w3.org/TR/SVG/paths.html#DProperty).

To do so, you can calculate the midpoint of the two points by simply taking the average x and y values and draw a line using the Q quadratic Bezier curve command. For example, if x1y1 is your start point and x2y2 is your end point, calculate the middle x3y3 and draw it like so: d='M x1,y1 Q x2,y2 x3,y3'