Logarithmic bar chart using Protovis

Question

var vis = new pv.Panel().canvas('grphLangSpeakers').height(langCount*(barWidth+barGap)).width(canvasWidth)
    .add(pv.Bar)
    .data(popCountArray)
    .bottom(0).width(function(d){ return d})
    .height(barWidth)
    .top(function() {return this.index * (barGap+barWidth)});

vis.render();

I'm using above code to generate a Bar Chart using Protovis. How to change this chart to Logarithmic Scale?