Zoomable Area Canvas for Retina

forked from interwebjill's block: Zoomable Area SVG

which was forked from mbostock's block: Zoomable Area

Zoom is not centered over the mouse cursor in retina.

<!DOCTYPE html>
<style>

/* to prevent browser overscrolling bounce */
  html {
    overflow: hidden;
  }

body {
    margin: 0;
  }

</style>
  <canvas></canvas>
<script src="https://d3js.org/d3.v4.min.js"></script><script>

var targetWidth = 960,
    targetHeight = 500;

var canvas = document.querySelector("canvas");
    context = canvas.getContext("2d");

var ratio = getHighDPICanvas(canvas),
    canvasWidth = targetWidth * ratio,
    canvasHeight = targetHeight * ratio;

canvas.width = canvasWidth;
canvas.height = canvasHeight;
canvas.style.width = `${targetWidth}px`;
canvas.style.height = `${targetHeight}px`;

var margin = {top: 20, right: 20, bottom: 30, left: 60},
    width = canvasWidth - margin.left - margin.right,
    height = canvasHeight - margin.top - margin.bottom;

var parseTime = d3.timeParse("%Y/%m/%d %H:%M");

var x = d3.scaleTime()
    .range([0, width]);

var x2 = d3.scaleTime()   // for zooming
    .range([0, width]);

var y = d3.scaleLinear()
    .range([height, 0]);

var area = d3.area()
    .x(function(d) { return x(d.date); })
    .y0(y(0))
    .y1(function(d) { return y(d.value); })
    .curve(d3.curveStep)
    .context(context);

// axes text variables
context.strokeStyle = "black";
context.font = '10pt sans-serif';
var tickCount = 10,
    tickSize = 6,
    tickPadding = 3;

d3.csv("TBLkW.csv", function(d) {
  d.date = parseTime(d.date);
  d.value = +Math.round(d.TBLkW);
  return d;
}, function(error, data) {
  if (error) throw error;

var xExtent = d3.extent(data, function(d) { return d.date; });
  x.domain(xExtent);
  x2.domain(x.domain());
  y.domain(d3.extent(data, function(d) { return d.value; }));

var xTicks = x.ticks(tickCount),
      xTickFormat = x.tickFormat(),
      yTicks = y.ticks(tickCount),
      yTickFormat = y.tickFormat(tickCount);

var zoom = d3.zoom()
  .scaleExtent([1, Infinity])
  .translateExtent([[0, 0], [width, height]])
  .extent([[0, 0], [width, height]])
    .on("zoom", zoomed);

d3.select("canvas").call(zoom);
  // d3.select("canvas").call(zoom.transform, d3.zoomIdentity);

context.translate(margin.left, margin.top);

function draw() {
    context.clearRect(-margin.left, -margin.top, canvas.width, canvas.height);

xAxis();
     yAxis();

context.save();

// clip path
     context.beginPath()
     context.rect(0, 0, width, height);
     context.clip();

context.beginPath();
     area(data);
     context.fillStyle = 'steelblue';
     context.fill();

// restore context without a clip path
     context.restore();
  }

draw();

function zoomed() {
    t = d3.event.transform;
    x = t.rescaleX(x2);
    draw();
  }

function xAxis() {
    context.beginPath();
    xTicks.forEach(function(d) {
      context.moveTo(x(d), height);
      context.lineTo(x(d), height + tickSize);
    });
    context.stroke();

context.beginPath();
    context.moveTo(0, height);
    context.lineTo(width, height);
    context.stroke();

context.textAlign = "center";
    context.textBaseline = "top";
    xTicks.forEach(function(d) {
      context.fillText(xTickFormat(d), x(d), height + tickSize);
    });

// context.save();
    //
    // // clip path
    // context.beginPath()
    // context.rect(0, height, width, height + tickSize);
    // context.lineWidth = 0;
    // context.strokeStyle = "none";
    // context.stroke();
    // context.clip();
    //
    // context.restore();
  }

function yAxis() {

context.beginPath();
    yTicks.forEach(function(d) {
      context.moveTo(0, y(d));
      context.lineTo(-6, y(d));
    });
    context.stroke();

context.beginPath();
    context.moveTo(-tickSize, 0);
    context.lineTo(0.5, 0);
    context.lineTo(0.5, height);
    context.lineTo(-tickSize, height);
    context.stroke();

context.textAlign = "right";
    context.textBaseline = "middle";
    yTicks.forEach(function(d) {
      context.fillText(yTickFormat(d), -tickSize - tickPadding, y(d));
    });

context.save();
    context.rotate(-Math.PI / 2);
    context.textAlign = "right";
    context.textBaseline = "top";
    context.fillText("Power (kW)", -10, 10);
    context.restore();
  }

});

function getHighDPICanvas(c) {
    var devicePixelRatio = window.devicePixelRatio || 1;
    var backingStoreRatio = [
        c.webkitBackingStorePixelRatio,
        c.mozBackingStorePixelRatio,
        c.msBackingStorePixelRatio,
        c.oBackingStorePixelRatio,
        c.backingStorePixelRatio,
        1
    ].reduce(function(a, b) { return a || b });

return devicePixelRatio / backingStoreRatio;
}

</script>

https://d3js.org/d3.v4.min.js