hen ( function(data) {

  // List of subgroups = header of the csv files = soil condition here
  const subgroups = data.columns.slice(1)

  // List of groups = species here = value of the first column called group -> I show them on the X axis
  const groups = data.map(d => d.group)

  // Add X axis
  const x = d3.scaleBand()
      .domain(groups)
      .range([0, width])
      .padding([0.2])
  svg.append("g")
    .attr("transform", `translate(0, ${height})`)
    .call(d3.axisBottom(x).tickSizeOuter(0));

 