TSVB table trend arrows always show slope of 0

[flash1293]

Kibana version: master

Elasticsearch version:

Server OS version:

Browser version:

Browser OS version:

Original install method (e.g. download page, yum, from source, etc.):

Describe the bug: The trend arrows in the TSVB table visualization should indicate the slope of a linear regression of the given metric over time, but they always show a 0.

Steps to reproduce:

1. Create TSVB vis
2. Go to table
3. Select group by field
4. Enable trend arrows for metric

Expected behavior:

Trend arrows should be shown for each line respective to slope, but slope is always zero.

Screenshots (if relevant):

Response showing a timeseries consisting out of two values clearly having a positive linear regression, but the slope value is still zero. (Value is calculated in src/plugins/vis_type_timeseries/server/lib/vis_data/table/process_bucket.js)

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

This might very well be me misunderstanding how this feature is supposed to work, but if that's the case it's probably worth mentioning it in the docs.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[sulemanof]

Hi @flash1293 !
Took a look into this issue and have the next feelings:

1. The lib regression, which is used only for slope calculating in TSVB across whole Kibana, seems to be not maintained for a long time. I think this is a good point to replace it with raw slope calculating without any lib.
2. I have the same confusion about how this feature is supposed to work. FMPOV it should show the slope of the last bucket only, since TSVB table shows data from the last bucket. In that case the calculating formula will be really simple, and the trend arrow would be more demonstrative.

[flash1293]

@sulemanof I agree to both of your points and think it's the right thing to do - I'm just a little concerned we are changing existing behavior (even if it's broken in the last few versions).

@timroes Do you think that's OK? We could add a breaking change to the changelog (like in #65467) for the unlikely case someone will run into this (given the absence of community bug reports).

Either way we should add a section to the docs documenting the behavior together with the bug fix.

[timroes]

I would agree with everything said here and agree that we can break that with an appropriate notice, since I don't see how anyway would have used that reasonable in that current way, and add: Please let's convert the code to TypeScript when we're anyway inlineing it into the repository :)

[flash1293]

If we go with 2. we don't need to inline anything, the "trend" will just be ((currentValue - previousValue) / currentValue). @sulemanof will you provide a fix?