d3article-en5.3

function zoomed() {
  const transformation = d3.event.transform;

  const rightEdge = Math.abs(transformation.x) / transformation.k + width / transformation.k;
  const bottomEdge = Math.abs(transformation.y) / transformation.k + height / transformation.k;

  if (rightEdge > width) {
    transformation.x = -(width * transformation.k - width);
  }

  if (bottomEdge > height) {
    transformation.y = -(height * transformation.k - height);
  }

  rescaledX = transformation.rescaleX(x);
  rescaledY = transformation.rescaleY(y);

  xAxisElement.call(xAxis.scale(rescaledX));
  yAxisElement.call(yAxis.scale(rescaledY));

  linesContainer.selectAll('path')
    .attr('d', regionId => {
      return d3.line()
        .defined(d => d.percent !== 0)
        .x(d => rescaledX(d.date))
        .y(d => rescaledY(d.percent))(regions[regionId].data);
    });

  voronoiGroup
    .attr('transform', transformation);
}