Right now, the bars are black. A linear scale will interpolate between colors, just like a regular number. Add the following to the end of the AJAX callback:
var yDomain = d3.extent(data, function(datum, index){
return datum.count;
})
var colorScale = d3.scaleLinear();
colorScale.domain(yDomain)
colorScale.range(['#00cc00', 'blue'])
d3.selectAll('rect')
.attr('fill', function(datum, index){
return colorScale(datum.count)
})Notice that we calculate they Domain using d3.extent so that the real minimum of the data set is used to map #00cc00:
