saving individual parts of data join

data-join.js

// create single idempotent container that is appended to the DOM only once
const container = selection.selectAll('.lines').data([null]).enter().append('g').attr('class', 'lines')

// bind data, create and save the update selection
let update = container.selectAll('.line')
    .data(data, d => d.id)

// create and save the exit selection
let exit = update.exit()

// do something with a selection
exit
  .remove()

// create and save enter selection
let enter = update
    .enter()

// do something with enter selection
enter
    .append('path')
    .attr('class', 'line')

// create and save combined selection
let merged = update
    .merge(enter)