time scatter plot w/ chart.js

Question

I'm trying to render a scatter plot in chart.js of (x,y) data where x is a date string. I've seen many examples and tutorials online where the instructor uses a function to generate the time stamp for an example chart, but I haven't found any examples that use real data like one might collect.

I have data that looks like this (collected from cron):

2017-07-08T06:15:02-0600,23.375
2017-07-08T06:20:02-0600,23.312
2017-07-08T06:25:02-0600,23.312
2017-07-08T06:30:02-0600,23.25

I tried a data like this in chart.js (both with and without "quotes" around the data string):

data: [{
  x: 2017-07-08T06:15:02-0600,
  y: 23.375
},{
  x: 2017-07-08T06:20:02-0600,
  y: 23.312
},{
  x: 2017-07-08T06:25:02-0600,
  y: 23.312
},{
  x: 2017-07-08T06:30:02-0600,
  y: 23.25

}],

Nothing renders. What am I doing wrong?

Answer 1

According to the documentation of scatter charts:

Unlike the line chart where data can be supplied in two different formats, the scatter chart only accepts data in a point format.

So you can't use values like 2017-07-08T06:15:02-0600. You can convert dates into numbers and use them in your data.

In your case:

data: [{
        x: 1499516102000,
        y: 23.375
    }, {
        x: 1499516402000,
        y: 23.312
    }, {
        x: 1499516702000,
        y: 23.312
    }, {
        x: 1499517002000,
        y: 23.25
    }
]

Now your xAxes will be with numbers, so you can use a callback to modify xAxes labels.

Answer 2

That advice isn't quite right. The javascript moment.js makes it possible to plat scatter data using dates as the x axis value. For some reason the bundled version in Chart.bundle.js wasn't working for me, so I downloaded moment.js directly. I'm using this to setup:

<script src="js/moment.js"></script>
<script src="js/Chart.min.js"></script>

and this for my chart.js data details:

data: [
  {x: moment("2017-07-08T06:15:02-0600"), y: 23.375},
  {x: moment("2017-07-08T06:20:02-0600"),y: 23.312},
  {x: moment("2017-07-08T06:25:02-0600"),y: 23.312},
  {x: moment("2017-07-08T06:30:02-0600"),y: 23.25}
],

It's working great!

Answer 3

Another solution that worked great for me, was to just use the line type for the chart, configure it for using dates for the x axis, and addtionally disable the lines, so that it just looks like a scatterplot.

new Chart(ctx, {
        type: 'line',
        data: {
            datasets: [{
                x: 2017-07-08T06:15:02-0600,
                y: 23.375
              },{
                x: 2017-07-08T06:20:02-0600,
                y: 23.312
              },{
                x: 2017-07-08T06:25:02-0600,
                y: 23.312
              },{
                x: 2017-07-08T06:30:02-0600,
                y: 23.25
            }]
          },
          options: {
            showLine: false,
            scales: {
              x:{
                type: 'time',
                display: true,
                title: {
                  display: true,
                  text: 'Date'
                },
              },
            }
          }
       }
    );

I see this as a quite elegant solution. The documentation even specifies:

The scatter chart supports all of the same properties as the line chart. By default, the scatter chart will override the showLine property of the line chart to false.