Invisible line on series when using "Offset series time by" on TSVB with series Stacked

[Kilhog]

Kibana version: 7.15.2
Elasticsearch version: 7.15.2
Server OS version: Latest Docker
Browser version: Safari
Browser OS version: 15.1
Original install method (e.g. download page, yum, from source, etc.): Docker
Describe the bug:

When I try do make a TSBV with my current data on 1 year (01/01/2021 - 31/12/2021), and add a second series on the same graph with the same parameters except the "Offset series time by" set to 1y, I have a weird behaviour. The line of the first series (without time shift) is not displayed, but I can still view the data with the "popover dialog"

Steps to reproduce:

1. Create TSVB with a first series (cumulative sum, grouped by terms, with Chart type: line and Stacked: stacked)
2. Create a second series (cumulative sum with no grouped by, with Chart type: line and Stacked: none and time shift 1y)
3. Use time range from 01/01 to 31/12
4. Offset the second series by 1 year

PS : It's maybe related to a other issue I reported #120449, the only different between the two behaviour is the time range. And another hint, if I set the graph type to "bar" the bar correctly show up.

Expected behavior:

Chart line correctly draw

Screenshots (if relevant):

Graph bug :

(Graph with bars instead of lines)

First series configuration :

Second series configuration :

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)

[stratoula]

@Kilhog we could also reproduce this. We are not 100% sure why this happens. The problem is that the timeseries interval changes from 24hours to 7days. If you change the interval to 1d, you will be able to see your series

[Kilhog]

@stratoula Yes indeed, with 1d interval I see the series

[flash1293]

After investigation, the problem happening here is that the shift is not compatible to the interval (a year is not evenly dividable by weeks) which causes a chart with gaps between data points. For the stacked area chart these gaps cause the series to not get rendered.

The following things can be done to work around this problem:

• Make sure the interval and the time shift are compatible by setting the interval (in this case by setting the interval to daily)
• Make sure the interval and the time shift are compatible by setting the shift (in this case using 52 weeks instead of 1y)
• Switch to Lens - besides the two workaround stated above, Lens allows you to configure the way gaps in the chart are handled and it's possible to interpolate between them

[flash1293]

Discussed this offline and we decided to not extend TSVB with additional features to render the line differently for situations like this. However, we will soon get to #117280 which allows you to do the same thing in Lens. Together with #116424 it will be very easy to migrate existing configurations over.

[flash1293]

Blocked on #117280

[flash1293]

As #117280 has been merged, this is now possible to do in Lens. To make it work with TSVB, see the comment above #120870 (comment)