Timestamp '1970/01/01 00:00:00' formats as 'undefined' in hovertext

[adamjhawley]

I tried to customise the hovertext format of some timestamp data and found that the timestamp '1970/01/01 00:00:00' formats as 'undefined'.

Note: please hover over the first data point in the example below to see the issue I am describing.

Minimal example to recreate the issue (CodePen here):

var dates = [0,1].map(x => `1970-01-01 00:00:0${x}`);

var data = [
  {
    type: 'scatter',
    x: dates,
    y: [5,2,3,1,4],
    hovertemplate: "%{x}, %{y}"
  }]

var layout = {
  xaxis: {
    tickformat: '%H:%M:%S',
    hoverformat: '%H:%M:%S'
  },
}

Plotly.newPlot('myDiv', data, layout);

I am happy to try and fix this issue myself but could benefit from someone pointing me in the right direction. I presume that the issue is caused by something like "0 seconds since epoch" is wrongly resolved as falsey at some point.

Thanks in advance for any pointers/help!

[archmoj]

Nice find 🥇
After long investigation,
it looks like the bug is coming from this block:

plotly.js/src/plots/cartesian/axes.js

Lines 1770 to 1779 in 630b092

	// can we remove the whole time part?
	if(dateStr === '00:00:00' || dateStr === '00:00') {
	dateStr = headStr;
	headStr = '';
	} else if(dateStr.length === 8) {
	// strip off seconds if they're zero (zero fractional seconds
	// are already omitted)
	// but we never remove minutes and leave just hours
	dateStr = dateStr.replace(/:00$/, '');
	}

Perhaps we should modify the first if statement to be something like:

if(headStr !== undefined && (dateStr === '00:00:00' || dateStr === '00:00')) {

I am looking forward to reviewing your pull request.
FYI - Our contributing information is at https://github.com/plotly/plotly.js/blob/master/CONTRIBUTING.md

[adamjhawley]

Thanks @archmoj for the pointers! I have now created PR #6752.

In addition to your suggestion, I had to add another change which prevents the function from trailing stripping zeroes when they were not added because of 'extraPrecision'.