Plotly.js bar add secondary text for x-axis bars

I am trying to implement the following chart

I tried the following:

data: Plotly.Data[] = [
    {
      type: 'bar',
      x: ['GHS 1', 'GHS 2', 'GHS 3', 'GHS 4', 'GHS 5'],
      y: [3, 4, 20, 15, 6],
      width: 0.7,
      marker: {
        color: [
          '#FF0000',
          '#FFB340',
          '#FFD940',
          '#95BC28',
          '#009933'
        ]
      }
    }
];

layout = {
    yaxis: {
      showgrid: false,
      showline: true,
      showticklabels: false,
      automargin: true,
      title: {
        text: 'Class Probability',
        standoff: 0
      }
    },
    xaxis: {
    },
    font: { size: 16 },
    width: '100%',
    height: '100%'
  };

Output:

How can I add a secondary text under each x-axis bar?

Plotly allows to use <br> tags for the labels that are set in x. Each label has its corresponding <text> element in the DOM, given the string 'a <br> b', it will create 2 <tspan> (subtext) elements for each part 'a' and 'b', which allows to create CSS rules for each distinct part (ie. so we can adjust the font size for the secondary text) :

.xaxislayer-above .xtick text tspan:not(:first-child) {
  font-size: 13px;
}

const x = ['GHS 1', 'GHS 2', 'GHS 3', 'GHS 4', 'GHS 5'];

const range = ['< 5', '5-50', '50-300', '300-2000', '> 2000'];
const unit = 'mg/kg';

const xlabels = x.map((xi, i) => `${xi}<br><br>ALD ${range[i]}<br>${unit}`);

You can assign xlabels to x data directly. Or you can keep them (x's) "clean" and assign the full labels as ticktext in the layout :

  xaxis: {
    tickmode: 'array',
    tickvals: x,
    ticktext: xlabels
  }

Note that it will also override the xs in the hover data (even the second method, which is thus less useful than expected..). You will probably need to set hoverinfo: 'text' and define a custom array in hovertext (or use hovertemplate).