How to draw Horizontal line on Bar Chart Chartjs

问题:

I have the following script of drawing bar chart and I wanna add horizontal line on particular y dot. I was trying following example link and I just substituted Chart.types.Line.extend with Chart.types.Bar.extend but as a result I'm getting can not read property extend of undefined So can someone help to implement above example which in the link properly or suggest another decision

my source code without horizontal line

  var ctx = document.getElementById("myChart");
        var myChart = new Chart(ctx, {
            type: 'bar',
            data: {
                labels: ["Red", "Blue", "Yellow", "Green", "Purple", "Orange"],
                datasets: [{
                    label: '# of Votes',
                    data: [12, 19, 3, 5, 2, 3],
                    backgroundColor: [
                        'rgba(255, 99, 132, 0.2)',
                        'rgba(54, 162, 235, 0.2)',
                        'rgba(255, 206, 86, 0.2)',
                        'rgba(75, 192, 192, 0.2)',
                        'rgba(153, 102, 255, 0.2)',
                        'rgba(255, 159, 64, 0.2)'
                    ],
                    borderColor: [
                        'rgba(255,99,132,1)',
                        'rgba(54, 162, 235, 1)',
                        'rgba(255, 206, 86, 1)',
                        'rgba(75, 192, 192, 1)',
                        'rgba(153, 102, 255, 1)',
                        'rgba(255, 159, 64, 1)'
                    ],
                    borderWidth: 1
                }]
            },
            options: {
                scales: {
                    yAxes: [{
                        ticks: {
                            beginAtZero:true
                        },


                    }]
                },

            }
        });

回答1:

You can use Chart.js plugins to do that. Plugins let you handle specific events such as beforeUpdate or afterDraw and are also easy to implement :

Chart.pluginService.register({
    afterDraw: function(chart) {
        // Code here will be triggered ... after the drawing
    }
});

An easy way to do it is to simply draw a line like you would you on a simple canvas element, after everything is drawn in your chart, using the lineTo method.

---

Here is a small example (and its related code) of how it would look like :

回答2:

With the answer from @tektiv, your yAxis always starts at 0.

This is a working example without the use of yAxe.min, so you can use it (for example, with beginAtZero:false) and the yAxe scales automatically with your data.

Line plugin:

var canvas = document.getElementById("barCanvas");
var ctx = canvas.getContext('2d');

Chart.pluginService.register({
    afterDraw: function(chart) {
        if (typeof chart.config.options.lineAt != 'undefined') {
            var lineAt = chart.config.options.lineAt;
            var ctxPlugin = chart.chart.ctx;
            var xAxe = chart.scales[chart.config.options.scales.xAxes[0].id];
            var yAxe = chart.scales[chart.config.options.scales.yAxes[0].id];
            ctxPlugin.strokeStyle = "red";
            ctxPlugin.beginPath();
            lineAt = yAxe.getPixelForValue(lineAt);
            ctxPlugin.moveTo(xAxe.left, lineAt);
            ctxPlugin.lineTo(xAxe.right, lineAt);
            ctxPlugin.stroke();
        }
    }
});

Chart:

var myChart = new Chart(ctx, {
    type: 'bar',
    data: {
        labels: ["Red", "Blue", "Yellow", "Green", "Purple", "Orange"],
        datasets: [{
            label: '# of Votes',
            data: [12, 19, 3, 5, 2, 3],
            backgroundColor: [
                'rgba(255, 99, 132, 0.2)',
                'rgba(54, 162, 235, 0.2)',
                'rgba(255, 206, 86, 0.2)',
                'rgba(75, 192, 192, 0.2)',
                'rgba(153, 102, 255, 0.2)',
                'rgba(255, 159, 64, 0.2)'
            ],
            borderColor: [
                'rgba(255,99,132,1)',
                'rgba(54, 162, 235, 1)',
                'rgba(255, 206, 86, 1)',
                'rgba(75, 192, 192, 1)',
                'rgba(153, 102, 255, 1)',
                'rgba(255, 159, 64, 1)'
            ],
            borderWidth: 1
        }]
    },
    options: {
       lineAt: 14,
       scales: {
            yAxes: [{
                ticks: {
                    beginAtZero:false
                }
            }]
        },
    }
});