Fill / Shade a chart above a specific Y value in PlotlyJS

Here is another solution exploiting Plotly's fill: "toself". The idea is to create a closed line trace which encloses the area above the threshold and the markers of the main line. Works for threshold values above zero and for numerical x-values.

The helper traces have their legend hidden and are grouped with the main trace, thereby preventing ugly artifacts when toggling the legend.

The function checks for each x-y-pair if the y-value is above the threshold, if yes

• check if there is already a segment above the threshold and use this one OR create a new sgement
• the segement starts from the y-value of the threshold and the intermediate x-value from the point above the threshold and the one before.
• each segment is terminated with an y-value which is equal to the threshol and the x-value which the mean of the last point in the segment and the next one

The function itself can be surely written in a nicer way but it's just a proof-of-concept.

function dataToTraces(data, threshold) {

  var fillers = [];

  var emptyFiller = {
    x: [],
    y: [],
    fill: "toself",
    mode: "lines",
    line: {
      width: 0
    },
    opacity: 0.5,
    fillcolor: "#8adcb3",
    showlegend: false,
    legendgroup: "main"
  }
  
  fillers.push(emptyFiller);

  for (var i = 0; i < data.y.length; i += 1) {
    if (data.y[i] >= threshold) {
      if (i !== 0 && data.y[i - 1] < threshold) {
        fillers[fillers.length - 1].x.push(data.x[i - 1] + (threshold - data.y[i - 1]) / (data.y[i] - data.y[i - 1]));
        fillers[fillers.length - 1].y.push(threshold);
      }
      fillers[fillers.length - 1].x.push(data.x[i]);
      fillers[fillers.length - 1].y.push(data.y[i]);
    } else if (fillers[fillers.length - 1].x.length > 0) {
      if (i !== 0 && data.y[i - 1] !== threshold) {
        fillers[fillers.length - 1].x.push(data.x[i - 1] + (threshold - data.y[i - 1]) / (data.y[i] - data.y[i - 1]));
        fillers[fillers.length - 1].y.push(threshold);
      }
      fillers.push(emptyFiller);
    }
  }
  return fillers;
}

var data = [{
  x: [0, 1, 2, 3, 4, 5, 6, 7, 8],
  y: [1, 3, 6, 2, -1, 5, 1, 3, 0],
  name: "main",
  legendgroup: "main"
}];
var fillers = dataToTraces(data[0], 2);

Plotly.newPlot("myDiv", data.concat(fillers));

<div id="myDiv"></div>
<script src="https://cdn.plot.ly/plotly-latest.min.js"></script>

A solution is to use multiple traces.
Split all your traces between ones which are above 0 and ones which are not.
When you are done you can fill them (or not) with the 'tozeroy' value.

The following jsfiddle shows a working example.

The code is as following :

HTML:

<div id="myDiv" style="width:600px;height:250px;"></div>

JS:

var data = [
  {
    x: ['A', 'B', 'C', 'D'],
    y: [1, 3, 6, 0],
    fill: 'tozeroy',
    fillcolor: '#8adcb3'
  },
  {
    x: ['D', 'F', 'G', 'I'],
    y: [0, -3, -2, 0],
    fill: 'toself'
  },
   {
    x: ['I', 'J', 'K'],
    y: [0, 5, 7],
    fill: 'tozeroy',
    fillcolor: '#0adcb3'
  }
];

Plotly.newPlot('myDiv', data);

The result looks as following :