Draw horizontal line on chart in chart.js on v2

Question

I have drawn a line chart using chart.js. For the labels and datasets i am getting values from the database. I am new to chart.js and its very powerful library, yet i am unable to completely understand it. I want to draw multiples horizontal lines. Like where if mean of dataset, standard deviation and min and max. I have tried the question here in stackoverflow but these are giving errors or may be i am not able to understand the working. This is my chart.js code

function display_graph(id, label, data) {
var ctx = document.getElementById(id);
var data = {
    labels: data.labels,
    datasets: [
        {
            label: label,
            fill: false,
            lineTension: 0.1,
            backgroundColor: "rgba(75,192,192,0.4)",
            borderColor: "rgba(75,192,192,1)",
            borderCapStyle: 'butt',
            borderDash: [],
            borderDashOffset: 0.0,
            borderWidth: 1,
            borderJoinStyle: 'miter',
            pointBorderColor: "rgba(75,192,192,1)",
            pointBackgroundColor: "#fff",
            pointBorderWidth: 1,
            pointHoverRadius: 5,
            pointHoverBackgroundColor: "rgba(75,192,192,1)",
            pointHoverBorderColor: "rgba(220,220,220,1)",
            pointHoverBorderWidth: 2,
            pointRadius: 1,
            pointHitRadius: 10,
            data: data.assay_value,
            spanGaps: false
        }
    ]
};

//options
var options = {
    responsive: true,
    title: {
        display: true,
        position: "top",
        text: label,
        fontSize: 18,
        fontColor: "#111"
    },
    legend: {
        display: true,
        position: "bottom",
        labels: {
            fontColor: "#333",
            fontSize: 16
        }
    }
};
var Blanks_Chart=null;
Blanks_Chart = new Chart(ctx, {
    type: 'line',
    data: data,
    options: options
});}

Answer 1

You could use the chart.js annotation plugin to easily draw lines on your chart without having to mess with rendering pixels in your canvas manually (the old approach that is giving you errors). Note, the plugin is created/supported by the same team as chart.js and is mentioned in the chart.js docs.

Here is an example codepen demonstrating creating a line on a chart.

Once you add the plugin, you simply just set annotation properties in your chart config. Here is an example.

var myChart = new Chart(ctx, {
  type: 'line',
  data: {
    labels: ["January", "February"],
    datasets: [{
      label: 'Dataset 1',
      borderColor: window.chartColors.blue,
      borderWidth: 2,
      fill: false,
      data: [2, 10]
    }]
  },
  options: {
    responsive: true,
    title: {
      display: true,
      text: 'Chart.js Draw Line On Chart'
    },
    tooltips: {
      mode: 'index',
      intersect: true
    },
    annotation: {
      annotations: [{
        type: 'line',
        mode: 'horizontal',
        scaleID: 'y-axis-0',
        value: 5,
        borderColor: 'rgb(75, 192, 192)',
        borderWidth: 4,
        label: {
          enabled: false,
          content: 'Test label'
        }
      }]
    }
  }
});

Answer 2

if you want to draw threshold line,easiest way is that using mixed line chart.

Note: Make an array filled with threshold value and the length should be same as your dataset.

var datasets = [1, 2, 3];
var ctx = document.getElementById('chart').getContext('2d');
var thresholdValue = 2;
var thresholdHighArray = new Array(datasets.length).fill(thresholdValue);
var myChart = new Chart(ctx, {
            type: 'line',
            data: {
                labels: [],
                datasets: [
                {datasets}, thresholdHighArray]
            },         
            options: {
                responsive: true,
                legend: {
                    position: 'bottom',
                },
                scales: {
                    xAxes: [{
                        display: true,
                        scaleLabel: {
                                display: true,
                                labelString: 'Readings'
                        }
                    }],
                    yAxes: [{
                        display: true,
                        scaleLabel: {
                                display: true,
                                labelString: 'Reading ( °C )'
                        }
                    }]
                },
                annotation: {
                  annotations: [
                    {
                      type: "line",
                      mode: "vertical",
                      scaleID: "x-axis-0",
                      borderColor: "red",
                      label: {
                        content: "",
                        enabled: true,
                        position: "top"
                      }
                    }
                  ]
                }
        });
    };

Answer 3

If you are using the NPM package chartjs-plugin-annotation.js, the important thing - which you may forget, is to register the plugin.

So first of all you installed the npm packages (here for React):

npm i react-chartjs-2                (depends on your framework)
npm i chartjs-plugin-annotation      (always required)

See Vue.js or Angular for their framework depending packages.

Option 1: Global plugin registration

import { Line } from 'react-chartjs-2';
import Chart from 'chart.js';
import * as ChartAnnotation from 'chartjs-plugin-annotation';

Chart.plugins.register([ChartAnnotation]); // Global

// ...
render() {
    return (
        <Line data={chartData} options={chartOpts} />
    )
}

Option 2: Per chart plugin registration

import { Line } from 'react-chartjs-2';
import * as ChartAnnotation from 'chartjs-plugin-annotation';

// ...
render() {
    return (
                                                   {/* per chart */}
        <Line data={chartData} options={chartOpts} plugins={[ChartAnnotation]} />
    )
}

chartData is equivalent to the data: { section and chartOpts to options: { from jordanwillis answer. See this github post for further information.

There are many other plugins available for chart.js.