You can extend the line chart to do this.
Preview
Script
Chart.defaults.NegativeTransparentLine = Chart.helpers.clone(Chart.defaults.line);
Chart.controllers.NegativeTransparentLine = Chart.controllers.line.extend({
update: function () {
// get the min and max values
var min = Math.min.apply(null, this.chart.data.datasets[0].data);
var max = Math.max.apply(null, this.chart.data.datasets[0].data);
var yScale = this.getScaleForId(this.getDataset().yAxisID);
// figure out the pixels for these and the value 0
var top = yScale.getPixelForValue(max);
var zero = yScale.getPixelForValue(0);
var bottom = yScale.getPixelForValue(min);
// build a gradient that switches color at the 0 point
var ctx = this.chart.chart.ctx;
var gradient = ctx.createLinearGradient(0, top, 0, bottom);
var ratio = Math.min((zero - top) / (bottom - top), 1);
gradient.addColorStop(0, 'rgba(75,192,192,0.4)');
gradient.addColorStop(ratio, 'rgba(75,192,192,0.4)');
gradient.addColorStop(ratio, 'rgba(0,0,0,0)');
gradient.addColorStop(1, 'rgba(0,0,0,0)');
this.chart.data.datasets[0].backgroundColor = gradient;
return Chart.controllers.line.prototype.update.apply(this, arguments);
}
});
and then
...
var myLineChart = new Chart(ctx, {
type: 'NegativeTransparentLine',
data: {
...
Fiddle – http://jsfiddle.net/g2r2q5Lu/