replace label with explanation on hover

explanation.js

// function to replace label with label's explanation on hover.
function mouseover() {
  const s = d3.select(this)[0][0];
  s.innerHTML = 'true positive rate';
  s.className = 'explanation';
}

function mouseout() {
  const s = d3.select(this)[0][0];
  s.innerHTML = 'Sensitivity';
  s.className = 'label';
}