Highcharts CSV export, incorrect date - highcharts

Question

I have an API which returns JSON data like this:
[
{
"name":"Something",
"data":[
{
"x":1541096421,
"y":2
},{
"x":1541436378,
"y":4
},{
"x":1553621371,
"y":2
}
]
},{
"name":"Something else",
"data":[
{
"x":1541096421,
"y":2
},{
"x":1541436378,
"y":4
},{
"x":1553621371,
"y":2
}
]
}
]
The x axis represents date/time and the y axis is a score. It's plotted on a chart like this, using some formatting to convert the date from millisecond timestamp to a readable date format:
function renderChart(data) {
$('#chartContainer').highcharts({
chart: {
type: 'scatter',
zoomType: 'xy'
},
title: {
text: chartTitle()
},
xAxis: {
allowDecimals: false,
title: {
text: 'Date completed',
scalable: false
},
type: 'datetime',
labels: {
formatter: function () {
if (true) {
return Highcharts.dateFormat('%d-%b-%y', moment.unix(this.value));
}
else {
if (this.value > 0 && this.value < 24) {
return this.value;
}
else
return 0;
}
}
},
tickPixelInterval: 100
},
yAxis: {
title: {
text: 'Score'
}
},
plotOptions: {
scatter: {
marker: {
radius: 5
}
}
},
series: data,
exporting: {
buttons: {
contextButton: {
menuItems: Highcharts.getOptions().exporting.buttons.contextButton.menuItems.filter(item => item !== 'openInCloud')
}
}
// Tried adding this but it doesn't make any difference:
/*,
csv: {
dateFormat: '%d/%m/%Y'
}*/
},
tooltip: {
formatter: function () {
return 'Score of <b>' + this.y + '</b> posted on <b>' + Highcharts.dateFormat('%d-%b-%y', moment.unix(this.x)) + '</b>';
}
}
});
}
This works fine. However, when I click 'export to CSV' in the Highchart graph on the front-end it outputs a CSV file where the date is always showing as "18/01/1970". Obviously it's something to do with the fact that the API is returning a timestamp value, but I don't see how I can modify the format in the CSV similar to how it's done in the chart rendering code.
Can anyone advise how (preferably without modifying the data returned by the API) to get the CSV to output a correct date in day/month/year format?
Many thanks

Answer 1

It can be done easily by wrapping Highcharts.Chart.prototype.getDataRows method and map the data array which is used for export. Check demo and code posted below.
Code:
(function(H) {
H.wrap(H.Chart.prototype, 'getDataRows', function(proceed, multiLevelHeaders) {
var rows = proceed.call(this, multiLevelHeaders);
rows = rows.map(row => {
if (row.x) {
row[0] = Highcharts.dateFormat('%d-%b-%y', row.x * 1000);
}
return row;
});
return rows;
});
}(Highcharts));
Demo:
https://jsfiddle.net/BlackLabel/yafx8cb1/1/
Docs:
https://www.highcharts.com/docs/extending-highcharts/extending-highcharts

Answer 2

As per #Core972's comment, the issue here is related to the timestamp in the API returning the date as seconds rather than milliseconds. I don't believe there is a way to manipulate the date format in the CSV export specifically so it will require a change to the API which returns the data.
Wojciech Chmiel's answer demonstrates how to override Highchart's output to re-format the date from a non-ideal source.