how to plot multiple time series in chartjs where each time series has different times

Question

I have two time series for example:

s1:
  2017-01-06 18:39:30    100
  2017-01-07 18:39:28    101

and

s2:
2017-01-07 18:00:00     90
2017-01-08 18:00:00    105

I want to plot these in a Chartjs chart, however it seems that Chartjs only takes one x-axis array (or label in Chartjs terminology).

So my question is what is the best way to plot both of these?

My approach was to write a function (in python, although the language doesn't really matter for this part) that iterates through both time series and creates 3 new arrays which is in the format apparently Chartjs needs based off the 1st example here: https://www.sitepoint.com/introduction-chart-js-2-0-six-examples/

The algorithm (in sudo code) goes like:

    # inputs are initial time series s1 and s2
    y1 = [] # to hold new s1 data values
    y2 = [] # to hold new s2 data values
    x  = [] # to hold times

    # iterate through longest series s1 or s2
    if s1[idx].time > s2[idx].time
      x.append(s1[idx].time)
      y1.append(s1[idx].data)
      # y2 appends the linear interpolation of 
      # of closest y2 points

    if (s1[idx].time < s2[idx].time)
      x.append(s2[idx].time)
      # opposite of above. ie. swap y1<->y2, s1->s2

    else # they have the same time
      x.append(s1[idx].time)
      y1.append(s1[idx].data)
      y2.append(s2[idx].data) 

There are a couple other conditional checks for when data runs out of the shorter series but that is the main logic. After which I have 3 arrays that I can now add to chart js via one time/label array/x-axis and two data arrays. However this seems WAY more complicated than it should be considering how common I assume this use case is. Any help or advice is greatly appreciated.

Answer 1

In ChartJS, label is a Category Cartesian Axis. Since you mentioned linear interpolation in your code, I assume the strings like 2017-01-06 18:39:30 are not categories, they represent the numeric values of the x-axis. So we need to inform ChartJS that the strings in the x axis are actually time. We do this in the scale options.

var s1 = {
  label: 's1',
  borderColor: 'blue',
  data: [
    { x: '2017-01-06 18:39:30', y: 100 },
    { x: '2017-01-07 18:39:28', y: 101 },
  ]
};

var s2 = {
  label: 's2',
  borderColor: 'red',
  data: [
    { x: '2017-01-07 18:00:00', y: 90 },
    { x: '2017-01-08 18:00:00', y: 105 },
  ]
};

var ctx = document.getElementById('myChart').getContext('2d');
var chart = new Chart(ctx, {
  type: 'line',
  data: { datasets: [s1, s2] },
  options: {
    scales: {
      xAxes: [{
        type: 'time'
      }]
    }
  }
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.20.1/moment.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.7.1/Chart.min.js"></script>
<canvas id="myChart"></canvas>

You can find more information in Chart.js documentation.

Answer 2

You can have data of the form [{x:"value", y:"value"}] when your graph is of type scatter.

So to make your graph work, do this.

var canvas = document.getElementById("graph");
var s1 = [{x:"2017-01-06 18:39:30",y:"100"},{x:"2017-01-07 18:39:28",y:"101"}];
var s2 = [{x:"2017-01-07 18:00:00",y:"90"},{x:"2017-01-08 18:00:00",y:"105"}];

var graphParams = {
	type:"scatter",
	data:{
		datasets: [{
			label:"Series 1",
			data:s1,
			borderColor:"red",
			backgroundColor:"transparent",
		},
		{
			label:"Series 2",
			data:s2,
			borderColor:"blue",
			backgroundColor:"transparent",
		}],
	},
	options:{
		maintainAspectRatio:false,
		responsive:false,	
		scales:{
			xAxes:[{
				type:"time",
				distribution: "series",
			}],
		}
	}

}
ctx = new Chart(canvas, graphParams);

<canvas id="graph" height="500" width="700"/>
<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.2.1/Chart.bundle.min.js"></script>