Format chart.js x labels

Question

I use chart.js with React. My question is how to show the month label (MMM) only once per month?

The chart currently has labels: [May 15, May 18, May 21, May 24, ...]
As result I want to get: [May 15, 18, 21, 24, 27, 30, Jun 2, 5, ...]

CodeSandbox

Line Chart:

import React from 'react'
import { Line } from 'react-chartjs-2'
import date from 'date-and-time'

const startDate = new Date(2020, 4, 15)

//===fake data===
const json = '{"responses":[{"rows":[{"values":["1"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["1"]},{"values":["6"]},{"values":["7"]},{"values":["5"]},{"values":["8"]},{"values":["9"]},{"values":["2"]},{"values":["1"]},{"values":["1"]},{"values":["1"]},{"values":["6"]},{"values":["3"]},{"values":["0"]},{"values":["20"]},{"values":["9"]},{"values":["3"]},{"values":["2"]},{"values":["1"]},{"values":["13"]},{"values":["3"]},{"values":["13"]},{"values":["13"]},{"values":["7"]},{"values":["12"]},{"values":["0"]}]}]}'
const values = JSON.parse(json).responses[0].rows.map((row, index) => {
  let date = new Date(2020, 4, 20)
  date.setDate(startDate.getDate() + index)
  return {
    y: row.values[0],
    x: date,
  }
})
//===============

const options = {
  legend: {
    display: false,
  },
  hover: {
    mode: 'index',
    intersect: false,
    animationDuration: 0,
  },
  scales: {
    yAxes: [{ position: 'right' }],
    xAxes: [{
      gridLines: { display: false },
      distribution: 'linear',
      type: 'time',
      time: {
        parser: 'MMM D',
        tooltipFormat: 'MMM D',
        unit: 'day',
        unitStepSize: 3,
        displayFormats: {
          day: 'MMM D',
        },
      },
      ticks: {
        min: startDate,
        max: date.format(date.addDays(new Date(), 1), 'MMM D'),
        autoSkip: true
      },
    }],
  },
  tooltips: {
    mode: 'x-axis',
  },
}

const data = {
  datasets: [
    {
      label: 'test',
      fill: false,
      data: values,
      backgroundColor: '#fff',
      borderWidth: 2,
      lineTension: 0,
      borderColor: 'forestgreen',
      hoverBorderWidth: 2,
      pointBorderColor: 'rgba(0, 0, 0, 0)',
      pointBackgroundColor: 'rgba(0, 0, 0, 0)',
      pointHoverBackgroundColor: '#fff',
      pointHoverBorderColor: 'forestgreen',
      showLine: true,
    }
  ],
}

const LineChart = () => <Line data={data} options={options}/>

export default LineChart

Answer 1

Solution 1 label filter: According to the filtering labels sample you can set a function to define what should be displayed:

options: {
    scales: {
        x: {
            display: true,
            ticks: {
                callback: function(dataLabel, index) {
                    // Apply logic to remove name of the month
                    return dataLabel
                }
            }
        },
        y: {
            display: true,
            beginAtZero: false
        }
    }
}

Github source of the example

Solution 2: You could prepare your labels array beforehand. Filter the occurence of all the upcoming mentions and feed this array to chart.js.

Answer 2

1. Define different displayFormats for day and month.
2. Enable ticks.major.
3. Mark the desired ticks as major through the afterBuildTicks callback.

time: {
  ...
  displayFormats: {
    day: 'D',
    month: 'MMM D',
  },
},
ticks: {
  major: {
    enabled: true
  }
},
afterBuildTicks: (scale, ticks) => {
  ticks.forEach((t, i) => t.major = i == 0 || new Date(t.value).getMonth() != new Date(ticks[i - 1].value).getMonth());
  return ticks;
}

Please take a look at your amended code and see how it works.

const startDate = new Date(2020, 4, 15)

//===fake data===
const json = '{"responses":[{"rows":[{"values":["1"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["0"]},{"values":["1"]},{"values":["6"]},{"values":["7"]},{"values":["5"]},{"values":["8"]},{"values":["9"]},{"values":["2"]},{"values":["1"]},{"values":["1"]},{"values":["1"]},{"values":["6"]},{"values":["3"]},{"values":["0"]},{"values":["20"]},{"values":["9"]},{"values":["3"]},{"values":["2"]},{"values":["1"]},{"values":["13"]},{"values":["3"]},{"values":["13"]},{"values":["13"]},{"values":["7"]},{"values":["12"]},{"values":["0"]}]}]}'
const values = JSON.parse(json).responses[0].rows.map((row, index) => {
  let date = new Date(2020, 4, 20);
  date.setDate(startDate.getDate() + index)
  return {
    y: row.values[0],
    x: date
  }
})

//===============

const options = {
  legend: {
    display: false
  },
  hover: {
    mode: 'index',
    intersect: false,
    animationDuration: 0
  },
  scales: {
    yAxes: [{
      position: 'right'
    }],
    xAxes: [{
      gridLines: {
        display: false
      },
      distribution: 'linear',
      type: 'time',
      time: {
        tooltipFormat: 'MMM D',
        unit: 'day',
        unitStepSize: 3,
        displayFormats: {
          day: 'D',
          month: 'MMM D',
        },
      },
      ticks: {
        major: {
          enabled: true
        }
      },
      afterBuildTicks: (scale, ticks) => {
        ticks.forEach((t, i) => t.major = i == 0 || new Date(t.value).getMonth() != new Date(ticks[i - 1].value).getMonth());
        return ticks;
      }
    }]
  },
  tooltips: {
    mode: 'x-axis',
  }
};

const data = {
  datasets: [{
    label: 'test',
    fill: false,
    data: values,
    backgroundColor: '#fff',
    borderWidth: 2,
    lineTension: 0,
    borderColor: 'forestgreen',
    hoverBorderWidth: 2,
    pointBorderColor: 'rgba(0, 0, 0, 0)',
    pointBackgroundColor: 'rgba(0, 0, 0, 0)',
    pointHoverBackgroundColor: '#fff',
    pointHoverBorderColor: 'forestgreen',
    showLine: true,
  }],
};

new Chart('myChart', {
  type: 'line',
  data: data,
  options: options
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.9.3/Chart.bundle.min.js"></script>
<canvas id="myChart" height="90"></canvas>