Draw the line use the same y values, plots line at the bottom

I have the same question with it

When use the same value in yAxis ，the plots line at the bottom

this is my code:

    var data = [{
            "creat_time" : "2013-03-19 10:28:30",
            "roundTripTime" : "32"
        }, {
            "creat_time" : "2013-03-19 09:07:12",
            "roundTripTime" : "45"
        }, {
            "creat_time" : "2013-03-19 08:57:09",
            "roundTripTime" : "26"
        }, {
            "creat_time" : "2013-03-19 08:47:10",
            "roundTripTime" : "90"
        }, {
            "creat_time" : "2013-03-19 08:37:12",
            "roundTripTime" : "80"
        }, {
            "creat_time" : "2013-03-19 08:27:08",
            "roundTripTime" : "36"
        }
    ]
    var data1 = [{
            "creat_time" : "2013-03-19 10:28:30",
            "roundTripTime" : "100"
        }, {
            "creat_time" : "2013-03-19 09:07:12",
            "roundTripTime" : "100"
        }, {
            "creat_time" : "2013-03-19 08:57:09",
            "roundTripTime" : "100"
        }, {
            "creat_time" : "2013-03-19 08:47:10",
            "roundTripTime" : "100"
        }, {
            "creat_time" : "2013-03-19 08:37:12",
            "roundTripTime" : "100"
        }, {
            "creat_time" : "2013-03-19 08:27:08",
            "roundTripTime" : "100"
        }
    ]
    function draw(data){

    var margin = {
        top : 20,
        right : 20,
        bottom : 30,
        left : 50
    };
    var width = 960 - margin.left - margin.right;
    var height = 500 - margin.top - margin.bottom;
    var parseDate = d3.time.format("%Y-%m-%d %H:%M:%S").parse;
    var x = d3.time.scale()
        .range([0, width]);

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

    var xAxis = d3.svg.axis()
        .scale(x)
        .orient("bottom")
        .tickFormat(d3.time.format("%H:%M"));

    var yAxis = d3.svg.axis()
        .scale(y)
        .orient("left");

    var line = d3.svg.line()
        .x(function (d) {
            return x(d.creat_time);
        })
        .y(function (d) {
            return y(d.roundTripTime);
        });

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

    data.forEach(function (d) {
        d.creat_time = parseDate(d.creat_time);
        d.roundTripTime = +d.roundTripTime;
    });

    data = sortByReturnTime(data);

    x.domain(d3.extent(data, function (d) {
            return d.creat_time;
        }));
    y.domain(d3.extent(data, function (d) {
            return d.roundTripTime;
        }));

    svg.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")")
    .call(xAxis);

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

    svg.append("path")
    .datum(data)
    .attr("class", "line")
    .attr("d", line);
    }
    draw(data);

When called

draw(data)

it's right.

the svg is:

And called

draw(data1) //the y values is the same

in the svg ,plots line at the bottom ,but if there are not errors, it should be at top.

the svg is:

Any idea what is going wrong here?what can I do something make the plots line at the top? Any help is appreciated.

---

The problem is that you're setting the input domain of your ranges using d3.extent . If all the values are the same, everything is going to map to a 0 y coordinate. To start the y scale at 0, replace

y.domain(d3.extent(data, function (d) {
        return d.roundTripTime;
    }));

with

y.domain([0, d3.max(data, function (d) {
        return d.roundTripTime;
    })]);

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