Solution explanation
As stated in this feature request on echarts' github, they plan to add what you're looking for in the future. But for now, it is still not supported.
So I found a workaround to have the tooltip display all series even if they don't have a value at the exact x where the axisPointer is. To do so, I used the tooltip formatter that can be defined as a callback function that is called every time the tooltip has to be changed (i.e. every time the axisPointer moves on a new value) and where you can specify your own tooltip format.
Inside this function, you have access to every piece of information about the data at the axisPointer (especially its xAxis value in our case). Given the xAxis value of the axisPointer, we can go through our series and find the closest value from that xAxis value.
formatter : (params) => {
//The datetime where the axisPointer is
var xTime = new Date(params[0].axisValue)
//Create our custom tooltip and add to its top the dateTime where the axisPointer is
let tooltip = `<p>${xTime.toLocaleString()}</p> `;
//Go through each serie
series.forEach((serie, index) => {
//Find the closest value
value = serie.data.reduce((prev, curr) => Math.abs(new Date(curr.value[0]).valueOf() - xTime.valueOf()) < Math.abs(new Date(prev.value[0]).valueOf() - xTime.valueOf()) ? curr : prev).value[1]
/* Add a line in our custom tooltip */
// Add the colored circle at the begining of the line
tooltip += `<p><span style="display:inline-block;margin-right:5px;border-radius:10px;width:9px;height:9px;background-color: ${myChart.getVisual({ seriesIndex: index }, 'color')}"></span>`
// Add the serie's name and its value
tooltip += `${serie.name}    <b>${value}</b></p>`;
});
return tooltip;
}
Full code
Here is the full code, using your example :
var myChart = echarts.init(document.getElementById('main'));
var series = [{
"name": "sensor 1",
//step: "end",
"data": [{
"value": [
"2019-02-20T11:47:44.000Z",
22.2
]
},
{
"value": [
"2019-02-20T12:03:02.000Z",
22.1
]
},
{
"value": [
"2019-02-20T12:18:19.000Z",
22.15
]
},
{
"value": [
"2019-02-20T12:33:36.000Z",
22.2
]
},
{
"value": [
"2019-02-20T12:48:53.000Z",
22.15
]
}
],
"type": "line"
},
{
"name": "sensor 2",
//step: 'end',
"data": [{
"value": [
"2019-02-20T11:47:44.000Z",
23.2
]
},
{
"value": [
"2019-02-20T12:03:02.000Z",
23.1
]
},
{
"value": [
"2019-02-20T12:22:19.000Z",
24.15
]
},
{
"value": [
"2019-02-20T12:39:36.000Z",
21.2
]
},
{
"value": [
"2019-02-20T12:52:53.000Z",
20.15
]
}
],
"type": "line"
}
]
option = {
legend: {},
tooltip: {
trigger: 'axis',
formatter : (params) => {
//The datetime where the axisPointer is
var xTime = new Date(params[0].axisValue)
//Create our custom tooltip and add to its top the dateTime where the axisPointer is
let tooltip = `<p>${xTime.toLocaleString()}</p> `;
//Go through each serie
series.forEach((serie, index) => {
//Find the closest value
value = serie.data.reduce((prev, curr) => Math.abs(new Date(curr.value[0]).valueOf() - xTime.valueOf()) < Math.abs(new Date(prev.value[0]).valueOf() - xTime.valueOf()) ? curr : prev).value[1]
/* Add a line in our custom tooltip */
// Add the colored circle at the begining of the line
tooltip += `<p><span style="display:inline-block;margin-right:5px;border-radius:10px;width:9px;height:9px;background-color: ${myChart.getVisual({ seriesIndex: index }, 'color')}"></span>`
// Add the serie's name and its value
tooltip += `${serie.name}    <b>${value}</b></p>`;
});
return tooltip;
}
},
xAxis: {
type: 'time'
},
yAxis: {
scale: true
},
series: series,
};
myChart .setOption(option)
<html>
<body>
<script src="https://cdnjs.cloudflare.com/ajax/libs/echarts/5.3.2/echarts.min.js"></script>
<div id="main" style="width: 600px; height:400px;"></div>
</body>
</html>
Further thoughts
Instead of displaying the value of the closest real point, we could also calculate the value with a simple linear interpolation.
Here is the formatter function with linear interpolation (not the most preformant, but working)
formatter : (params) => {
var xTime = new Date(params[0].axisValue)
let tooltip = `<p>${xTime.toLocaleString()}</p> `;
series.forEach((serie, index) => {
//Only works if series is chronologically sorted
prev_point = serie.data.reduce((prev, curr) => new Date(curr.value[0]).valueOf() <= xTime.valueOf() ? curr : prev)
next_point = serie.data.slice(0).reduce((prev, curr, i, arr) => {
if(new Date(curr.value[0]).valueOf() >= xTime.valueOf()) {
arr.splice(1);
}
return curr
})
var value = 0
if(next_point.value[1] == prev_point.value[1]){
value = next_point.value[1]
}
else {
//Linear interpolation
value = Math.round((prev_point.value[1] + (xTime.valueOf()/1000 - new Date(prev_point.value[0]).valueOf()/1000) * ((next_point.value[1] - prev_point.value[1]) / (new Date(next_point.value[0]).valueOf()/1000 - new Date(prev_point.value[0]).valueOf()/1000)))*10)/10
}
tooltip += `<p><span style="display:inline-block;margin-right:5px;border-radius:10px;width:9px;height:9px;background-color: ${myChart.getVisual({ seriesIndex: index }, 'color')}"></span> ${serie.name}    <b>${value}</b></p>`;
});
return tooltip;
}
- Display the series as steps
To make it visually more accurate, you can display the series as
step: 'end' and get the closest previous value instead of just the
closest value, using:
value = serie.data.reduce((prev, curr) => new Date(curr.value[0]).valueOf() <= xTime.valueOf() ? curr : prev).value[1]
Doing so, the value displayed in the tooltip will be exactly what you see on the graph.
The reduce() methods are slow on large datasets as they go through the whole series. For example, replacing it with binary search (dichotomic search) will drastically improve the performance.
I'd be interested if anyone has an idea to make it even more performant.
