Prevent d3.js area chart from dropping below 0 or x-Axis

Question

What would be causing this chart to have a value appear below the x-axis (0:00)?

My data looks like this (no values less than zero):

[{"x":1341806400,"y":4},
 {"x":1342411200,"y":0},
 {"x":1343016000,"y":0},
 {"x":1343620800,"y":7},
 {"x":1344225600,"y":6},
 {"x":1344830400,"y":73},
 {"x":1345435200,"y":328},
 {"x":1346040000,"y":0},
 {"x":1346644800,"y":0},
 {"x":1347249600,"y":0},
 {"x":1347854400,"y":0},
 {"x":1348459200,"y":11},
 {"x":1349064000,"y":17},
 {"x":1349668800,"y":0},
 {"x":1350273600,"y":0}]

Rendered Chart

The above chart is rendered via:

var margin = {top: 20, right: 30, bottom: 30, left: 40};
var width = max_width - margin.left - margin.right;
var height = 300; // + margin.top + margin.bottom;
var height_offset = 100;

var x = d3.time.scale()
    .range([0, width]);

var y = d3.scale.linear()
    .range([height, 0]);

var z = d3.scale.category20c();

var ticks_interval;
switch(this.period_type){
  case "day":
    ticks_interval = d3.time.days;
    break;
  case "week":
    ticks_interval = d3.time.weeks;
    break;
  case "month":
    ticks_interval = d3.time.months;
    break;
  case "year":
    ticks_interval = d3.time.years;
    break;
}

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom")
    .ticks(ticks_interval);

var yAxis = d3.svg.axis()
    .scale(y)
    .tickFormat(function(d){ 
       return numeral(d).format("00:00").replace(/^0:/,""); })
    .orient("left");

var stack = d3.layout.stack()
    .offset("zero")
    .values(function(d) { return d.values; })
    .x(function(d) { return d.date; })
    .y(function(d) { return d.value; });

var nest = d3.nest()
    .key(function(d) { return d.key; });

var area = d3.svg.area()
    .interpolate("cardinal")
    .x(function(d) { return x(d.date); })
    .y0(function(d) {return y(d.y0);})
    .y1(function(d) { return y(d.y0 + d.y); });

var svg = d3.select(chart_dom_el)
    .append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + height_offset)
    .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

var layers = stack(nest.entries(data));

x.domain(d3.extent(data, function(d){ return d.date; }));
y.domain([0,d3.max(data,function(d){
  return d.y0+ d.y;
})]);

//  re-map for formatted date
data = _.map(data,function(d){
  return {date: d.date.format("MM/D"),value:d.value};
});

svg.selectAll(".layer")
    .data(layers)
    .enter().append("path")
    .attr("class", "layer")
    .attr("d", function(d) { return area(d.values); })
    .style("fill", function(d, i) { return z(i); });

var x_axis_options = {x: 4, y: 9, rotate: 0};
if(data.length > 20){
  x_axis_options = {x: -27, y: 8, rotate: -45};
}else if(data.length > 13){
  x_axis_options = {y: -5, x: 27, rotate: 90};
}
svg.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")")
    .call(xAxis)
  .selectAll("text")
    .attr("y", x_axis_options.y)
    .attr("x", x_axis_options.x)
    .attr("transform", "rotate("+(x_axis_options.rotate)+")");

svg.append("g")
    .attr("class", "y axis")
    .call(yAxis);

Answer 1

This appears to have to do with the interpolation mode you used on the area generator - try changing from cardinal to linear, or some other area interpolation mode