create or convert a chart to display elements with percentages in d3.js

percentage.js

// percentage = value / total * 100
// store total in variable
var total = d3.sum(data, function(d) { return d.values; });

// set upper domain of y-axis not to 100% but to the percentage of the greatest value in your data
y.domain([0, d3.max(data, function(d) { return Math.round((d.values / total) * 100); })]);

// percentage in bar chart
var bars = d3.selectAll('.bar')
    .data(data)
  .enter().append('rect')
    .attr('class', 'bar')
    .attr('width', x.rangeBand())
    .attr('x', function(d) { return x(d.key); })
    .attr('y', function(d) { return y(Math.round((d.values / total) * 100)); })
    .attr('height', function(d) { return height - y(Math.round((d.values / total) * 100)); })
    .style('fill', color(colorKey));
    
// percentage in a tooltip
tooltip.html('Percent: ' + Math.round((d.values / total) * 100) + '%');