(Chart.js 3.7.0) change position of x-axis ticks to alternate between each tick? - callback

Question

I have a chart which effectively renders a timeline, but sometimes the tick positions are too close together
Current graph (padding: 25)
I can change the padding to be negative which puts them above the x-axis:
Ticks above x-axis (padding: -60)
But I'd prefer them to alternate to above and below eg 1st tick below, 2nd tick above, 3rd tick below etc.
Can I access the individual ticks padding to do this? See below my current code:
ticks: {
source: 'data',
maxRotation: 90,
minRotation: 90,
font: {
size: 12,
},
autoskip: true,
padding: -60,
},
TIA for any help! :-)

Answer 1

You can define two identical datasets together with two x-axes. One of the x-axes with position: 'top', the other one with default position ('bottom').
A slightly different ticks.callback function on both x-axes makes sure that only every second tick is displayed.
ticks: {
source: 'data',
...
callback: (v, i) => i % 2 ? v : ''
},
Please take a look at the following runnable code and see how it works.
const data = [
{ x: "2022-03-22", y: 0 },
{ x: "2022-04-01", y: 0 },
{ x: "2022-04-02", y: 0 },
{ x: "2022-04-03", y: 0 },
{ x: "2022-04-08", y: 0 },
{ x: "2022-04-12", y: 0 },
{ x: "2022-04-15", y: 0 }
];
new Chart('chart', {
type: 'line',
data: {
datasets: [{
data: data,
backgroundColor: 'black',
pointRadius: 5,
pointHoverRadius: 5,
borderWidth: 2,
xAxisID: 'x'
},
{
data: data,
backgroundColor: 'black',
pointRadius: 5,
pointHoverRadius: 5,
borderWidth: 2,
xAxisID: 'x2'
}]
},
options: {
plugins: {
legend: {
display: false
},
tooltip: {
callbacks: {
label: context => undefined
}
}
},
scales: {
y: {
ticks: {
display: false,
},
grid: {
display: false,
drawBorder: false
}
},
x: {
position: 'top',
type: 'time',
time: {
unit: 'day',
tooltipFormat: 'MMM DD'
},
ticks: {
source: 'data',
minRotation: 90,
callback: (v, i) => i % 2 ? v : ''
},
grid: {
display:false,
drawBorder: false
}
},
x2: {
type: 'time',
time: {
unit: 'day',
tooltipFormat: 'MMM DD'
},
ticks: {
source: 'data',
minRotation: 90,
callback: (v, i) => i % 2 ? '' : v
},
grid: {
display:false,
drawBorder: false
}
}
}
}
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/3.7.0/chart.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.29.1/moment-with-locales.js"></script>
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