score:8

Accepted answer

The below answer is for chartjs version 1.x

One way can be to create a custom graph that will allow for null values, in a previous question i answered i applied the same technique but it can also be used here.

In my answer, i explain how i have achieved the gaps https://stackoverflow.com/a/25319120/2737978

The only difference here is rather than having gaps in 1 datasets data, the gaps can be used to create your desired effect.

When setting up your data just make sure the two datasets lineup with the labels like so,

//labels that will applied to both datasets
labels: ["January", "February", "March", "April", "May", "June", "July"],

//data sets will lineup to the labels
data: [65, 34, 21, null, null, null, null]
data: [null, null, 88, 19, 86, 27, 90]

fiddle http://jsfiddle.net/leighking2/yoqfwt8o/

the extend line graph

 Chart.types.Line.extend({
        // Passing in a name registers this chart in the Chart namespace in the same way
        name: "MissingLine",
        initialize: function(data) {
            var helpers = Chart.helpers;
            //Declare the extension of the default point, to cater for the options passed in to the constructor
            this.PointClass = Chart.Point.extend({
                strokeWidth: this.options.pointDotStrokeWidth,
                radius: this.options.pointDotRadius,
                display: this.options.pointDot,
                hitDetectionRadius: this.options.pointHitDetectionRadius,
                ctx: this.chart.ctx,
                inRange: function(mouseX) {
                    return (Math.pow(mouseX - this.x, 2) < Math.pow(this.radius + this.hitDetectionRadius, 2));
                }
            });

            this.datasets = [];

            //Set up tooltip events on the chart
            if (this.options.showTooltips) {
                helpers.bindEvents(this, this.options.tooltipEvents, function(evt) {
                    var activePoints = (evt.type !== 'mouseout') ? this.getPointsAtEvent(evt) : [];
                    this.eachPoints(function(point) {
                        point.restore(['fillColor', 'strokeColor']);
                    });
                    helpers.each(activePoints, function(activePoint) {
                        activePoint.fillColor = activePoint.highlightFill;
                        activePoint.strokeColor = activePoint.highlightStroke;
                    });
                    this.showTooltip(activePoints);
                });
            }

            //Iterate through each of the datasets, and build this into a property of the chart
            helpers.each(data.datasets, function(dataset) {

                var datasetObject = {
                    label: dataset.label || null,
                    fillColor: dataset.fillColor,
                    strokeColor: dataset.strokeColor,
                    pointColor: dataset.pointColor,
                    pointStrokeColor: dataset.pointStrokeColor,
                    points: []
                };

                this.datasets.push(datasetObject);


                helpers.each(dataset.data, function(dataPoint, index) {
                    /**
                     *
                     * Check for datapoints that are null
                     */
                    if (helpers.isNumber(dataPoint) || dataPoint === null) {
                        //Add a new point for each piece of data, passing any required data to draw.
                        datasetObject.points.push(new this.PointClass({
                            /**
                             * add ignore field so we can skip them later
                             *
                             */
                            ignore: dataPoint === null,
                            value: dataPoint,
                            label: data.labels[index],
                            datasetLabel: dataset.label,
                            strokeColor: dataset.pointStrokeColor,
                            fillColor: dataset.pointColor,
                            highlightFill: dataset.pointHighlightFill || dataset.pointColor,
                            highlightStroke: dataset.pointHighlightStroke || dataset.pointStrokeColor
                        }));
                    }
                }, this);

                this.buildScale(data.labels);


                this.eachPoints(function(point, index) {
                    helpers.extend(point, {
                        x: this.scale.calculateX(index),
                        y: this.scale.endPoint
                    });
                    point.save();
                }, this);

            }, this);


            this.render();
        },

        draw: function(ease) {
            var helpers = Chart.helpers;
            var easingDecimal = ease || 1;
            this.clear();

            var ctx = this.chart.ctx;

            this.scale.draw(easingDecimal);


            helpers.each(this.datasets, function(dataset) {

                //Transition each point first so that the line and point drawing isn't out of sync
                //We can use this extra loop to calculate the control points of this dataset also in this loop

                helpers.each(dataset.points, function(point, index) {
                    point.transition({
                        y: this.scale.calculateY(point.value),
                        x: this.scale.calculateX(index)
                    }, easingDecimal);

                }, this);


                // Control points need to be calculated in a seperate loop, because we need to know the current x/y of the point
                // This would cause issues when there is no animation, because the y of the next point would be 0, so beziers would be skewed
                if (this.options.bezierCurve) {
                    helpers.each(dataset.points, function(point, index) {
                        //If we're at the start or end, we don't have a previous/next point
                        //By setting the tension to 0 here, the curve will transition to straight at the end
                        if (index === 0) {
                            point.controlPoints = helpers.splineCurve(point, point, dataset.points[index + 1], 0);
                        } else if (index >= dataset.points.length - 1) {
                            point.controlPoints = helpers.splineCurve(dataset.points[index - 1], point, point, 0);
                        } else {
                            point.controlPoints = helpers.splineCurve(dataset.points[index - 1], point, dataset.points[index + 1], this.options.bezierCurveTension);
                        }
                    }, this);
                }


                //Draw the line between all the points
                ctx.lineWidth = this.options.datasetStrokeWidth;
                ctx.strokeStyle = dataset.strokeColor;


                var penDown = false;
                var start = null

                helpers.each(dataset.points, function(point, index) {

                    /**
                     * no longer draw if the last point was ignore (as we don;t have anything to draw from)
                     * or if this point is ignore
                     * or if it's the first
                     */
                    if (!point.ignore && !penDown) {
                        ctx.beginPath();
                        penDown = true;
                        start = point;
                    }
                    if (index > 0 && !dataset.points[index - 1].ignore && !point.ignore) {
                        if (this.options.bezierCurve) {
                            ctx.bezierCurveTo(
                                dataset.points[index - 1].controlPoints.outer.x,
                                dataset.points[index - 1].controlPoints.outer.y,
                                point.controlPoints.inner.x,
                                point.controlPoints.inner.y,
                                point.x,
                                point.y
                            );
                        } else {
                            ctx.lineTo(point.x, point.y);
                        }

                    } else if (index === 0 || dataset.points[index - 1].ignore) {
                        ctx.moveTo(point.x, point.y);
                    }

                    if (((dataset.points.length > index + 1 && dataset.points[index + 1].ignore) ||
                        dataset.points.length == index + 1) && !point.ignore) {
                        ctx.stroke();

                        if (this.options.datasetFill) {
                            ctx.lineTo(point.x, this.scale.endPoint);
                            ctx.lineTo(start.x, this.scale.endPoint);
                            ctx.fillStyle = dataset.fillColor;
                            ctx.closePath();
                            if (point.x != start.x) {
                                ctx.fill();
                            }
                        }
                        penDown = false;
                    }

                }, this);


                //Now draw the points over the line
                //A little inefficient double looping, but better than the line
                //lagging behind the point positions
                helpers.each(dataset.points, function(point) {
                    /**
                     * don't draw the dot if we are ignoring
                     */
                    if (!point.ignore)
                        point.draw();
                });

            }, this);
        }
    });

then to use

var ctx = document.getElementById("chart").getContext("2d");
    var data = {
        labels: ["January", "February", "March", "April", "May", "June", "July"],
        datasets: [{
                label: "My First dataset",
                fillColor: "rgba(220,220,220,0.2)",
                strokeColor: "rgba(220,220,220,1)",
                pointColor: "rgba(220,220,220,1)",
                pointStrokeColor: "#fff",
                pointHighlightFill: "#fff",
                pointHighlightStroke: "rgba(220,220,220,1)",
                data: [65, 34, 21, null, null, null, null]
            },
            {
            label: "My Second dataset",
            fillColor: "rgba(151,187,205,0.2)",
            strokeColor: "rgba(151,187,205,1)",
            pointColor: "rgba(151,187,205,1)",
            pointStrokeColor: "#fff",
            pointHighlightFill: "#fff",
            pointHighlightStroke: "rgba(151,187,205,1)",
            data: [null, null, 88, 19, 86, 27, 90]
        }



        ]
    };
var myLineChart = new Chart(ctx).MissingLine(data);

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