Intersection point in line graph in Chart.js?

Question

Searched a lot about this but din\'t got any answer on this based on chart.js. I had already asked this question here in which I was using highchart.js and got the solution

Answer 1

Here we are )))

https://jsfiddle.net/Falseclock/5nbecn0z/

If you need to show intersections with X axis, then just imitate line with Y=0 https://jsfiddle.net/Falseclock/8g0ucdb1/

var ORDER_STATS = { 
"2016" : [10, 181, 194, -56, 130, 181, 179, 189, 30, 60, 193, 154], 
"2015" : [124, -50, 152, 187, 10, 164, 129, -16, 115, 119, 129, 171], 
"2014" : [-90, 80, 30, 59, 100, -30, 60, 116, 191, 181, -60, 106] 
};
var colors = ['206,191,26','119,206,26','26,200,206','236,124,98','206,26,140','26,77,206'];

// Definning X
var ordersChartData = {
    labels : ["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"],
    datasets : []
}

Object.keys(ORDER_STATS).forEach(function (key) {
    color = colors.shift();
    ordersChartData.datasets.push(
        {
            label: key,
            lineTension: 0,
            type: 'line',
            backgroundColor: "rgba("+color+",0.1)",
            borderColor: "rgba("+color+",1)",
            borderWidth: 2,
            pointBackgroundColor : "rgba("+color+",1)",
            pointBorderColor: "#fff",
            pointBorderWidth: 1,
            pointRadius: 4,
            pointHoverBackgroundColor : "#fff",
            pointHoverBorderColor: "rgba("+color+",1)",
            pointHoverBorderWidth: 1,
            data : ORDER_STATS[key]
        }
    );
});

var ctx = document.getElementById("myChart").getContext("2d");

Chart.defaults.global.defaultFontColor = 'grey';
Chart.defaults.global.defaultFontFamily = "Tahoma";
Chart.defaults.global.defaultFontSize = 11;
Chart.defaults.global.defaultFontStyle = 'normal';

var myChart = new Chart(ctx, {
    type: 'line',
    data: ordersChartData,
    defaultFontSize: 11,
    options: {
        responsive: true,

        title: {
            display: true,
            text: 'Intersection realization',
            fontColor: "#444",
            fontFamily: 'Tahoma',
            padding: 0
        },

        legend: {
            display: true,
            labels: {
                fontColor: 'grey',
                usePointStyle: true
            }
        },
        tooltips: {
            mode: "index",
            intersect: true,
            position: 'nearest',
            bodySpacing: 4
            
        }
    }
});

Chart.plugins.register({
    afterDatasetsDraw: function(chartInstance, easing) {

        var Y = chartInstance.scales['y-axis-0'];
        var X = chartInstance.scales['x-axis-0'];

        zeroPointY = Y.top + ((Y.bottom - Y.top) / (Y.ticks.length -1) * Y.zeroLineIndex);
        zeroPointX = Y.right;
        yScale = (Y.bottom - Y.top) / (Y.end - Y.start);
        xScale = (X.right - X.left) / (X.ticks.length - 1);

        var intersects = findIntersects(ORDER_STATS['2015'], ORDER_STATS['2014'] );
        var context = chartInstance.chart.ctx;
            
        intersects.forEach(function (result, idx) {
            context.fillStyle = 'red';
            context.beginPath();
            context.arc((result.x * xScale) + zeroPointX, (Y.end - Y.start) - (result.y * yScale) - ((Y.end - Y.start) - zeroPointY), 3, 0, 2 * Math.PI, true);
            context.fill();
        });
    }
});


function findIntersects(line1, line2)
{
    var intersects = [];
            
    line1.forEach(function(val,idx) {
        var line1StartX = idx;
        var line1StartY = line1[idx];
        var line1EndX = idx + 1;
        var line1EndY = line1[idx + 1];
        var line2StartX = idx;
        var line2StartY = line2[idx];
        var line2EndX = idx + 1;
        var line2EndY = line2[idx+1];
        
        result = checkLineIntersection(line1StartX, line1StartY, line1EndX, line1EndY, line2StartX, line2StartY, line2EndX, line2EndY);
        
        if (result.onLine1 && result.onLine2) {
            intersects.push(result);
        }
    });
    
    return intersects;
}

function checkLineIntersection(line1StartX, line1StartY, line1EndX, line1EndY, line2StartX, line2StartY, line2EndX, line2EndY) {
    // if the lines intersect, the result contains the x and y of the intersection (treating the lines as infinite) and booleans for whether line segment 1 or line segment 2 contain the point
    var denominator, a, b, numerator1, numerator2, result = {
        x: null,
        y: null,
        onLine1: false,
        onLine2: false
    };
    denominator = ((line2EndY - line2StartY) * (line1EndX - line1StartX)) - ((line2EndX - line2StartX) * (line1EndY - line1StartY));
    if (denominator == 0) {
        return result;
    }
    a = line1StartY - line2StartY;
    b = line1StartX - line2StartX;
    numerator1 = ((line2EndX - line2StartX) * a) - ((line2EndY - line2StartY) * b);
    numerator2 = ((line1EndX - line1StartX) * a) - ((line1EndY - line1StartY) * b);
    a = numerator1 / denominator;
    b = numerator2 / denominator;

    // if we cast these lines infinitely in both directions, they intersect here:
    result.x = line1StartX + (a * (line1EndX - line1StartX));
    result.y = line1StartY + (a * (line1EndY - line1StartY));
/*
        // it is worth noting that this should be the same as:
        x = line2StartX + (b * (line2EndX - line2StartX));
        y = line2StartX + (b * (line2EndY - line2StartY));
        */
    // if line1 is a segment and line2 is infinite, they intersect if:
    if (a > 0 && a < 1) {
        result.onLine1 = true;
    }
    // if line2 is a segment and line1 is infinite, they intersect if:
    if (b > 0 && b < 1) {
        result.onLine2 = true;
    }
    // if line1 and line2 are segments, they intersect if both of the above are true
    return result;
};

<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.5.0/Chart.js"></script>
<canvas id="myChart" width="650" height="241" style="display: block; width: 650px; height: 241px;"></canvas>