dygraph: show maximum and minimum

Question

My question is similar to Horizontal line to show average in dygraph, but not the same. I understand using underlayCallback but how do I get the underlying data to calculate maximum and minimum of the data. I was to highlight these point in the data, which is being read from a csv file like so: gplot.updateOptions({'file': filename}); and I do not know the filename in advance, so have to calculate the max,min on the fly. I feel it is somehow related to gplot.rawData_ but cannot figure out how.

Thanks.

Answer 1

You can query a Dygraph's data set to get the min and max using its getValue method, like so:

// Calculate the min/max y value in the Dygraph's data set.
function calcMinMax(g) {
  var ymin = g.getValue(0, 1), ymax = g.getValue(0, 1), v;
  for (var i = 0; i < g.numRows(); i++) {
    for (var j = 1; j < g.numColumns(); j++) {
      y = g.getValue(i, j);
      if (y < ymin) {
        ymin = y;
      } else if (y > ymax) {
        ymax = y;
      }
    }
  }
  return [ymin, ymax];
}

To draw the lines, you'll need to use an underlayCallback, e.g.

underlayCallback: function(ctx, area) {
  var mm = calcMinMax(this),
      ymin = mm[0],
      ymax = mm[1],
      canvasYmin = this.toDomYCoord(ymin),
      canvasYmax = this.toDomYCoord(ymax);
  ctx.strokeStyle= 'red';
  ctx.beginPath();
  ctx.moveTo(area.x, canvasYmin);
  ctx.lineTo(area.x + area.w, canvasYmin);
  ctx.moveTo(area.x, canvasYmax);
  ctx.lineTo(area.x + area.w, canvasYmax);
  ctx.closePath();
  ctx.stroke();       
}

See this jsbin for a working example.