Clarify that CPU is between samples and category&stack are at sample time

[squelart]

When CPU numbers are shown, the tooltip looks like this:

So it's something like this:

Category: [] Layout...
CPU Usage: 97% (...)
Stack:
parse_into()...
...

I'm concerned that we're mixing two "domains" of information, and it may give the wrong impression, e.g.: "Wow, this parse_into in layout is taking 97% of a CPU!".
But in fact, all we can say from this data is: Since the previous sample and now, we have used 97% of a CPU; And at this exact sample time we were calling parse_into in layout.

So I think we should clarify the difference.
Suggestion:

CPU Usage since previous sample: 97% (...)
.....................................................................................
Sample at time: 0.8465s
Category: [] Layout...
Stack:
parse_into()...
...

This clearly separates the more continuous CPU numbers from the point-like sample data.

As a bonus it shows the precise timestamp. I don't know how valuable that is, but for example it could help find markers at that time in the Marker Table.

┆Issue is synchronized with this Jira Task

[julienw]

It's been a long time I'd like to show the precise timestamps (including for markers actually).

Also I'm more generally 👍 for this change.

[mstange]

I filed #3179 with similar concerns but a different proposed solution.

[julienw]

@canova and what about this issue ?

[canova]

Yes, #3282 also fixes this issue. Closing.

[squelart]

I agree that #3282 helped, by visually separating CPU and sample information, great, thank you. 😁

However, I'm reopening this issue here, because I think it suggests a few more things that would help clarify the distinction further, and offer a bit more information:

• Clarify that the CPU usage is between two samples, e.g.: "80% (average between samples 36ms apart)". Optionally give the start timestamp, and maybe even the end. (This would be nice to show in marker tooltips as well; probably worth a separate issue if you agree.)
• Show the sample timestamp, new line "Sample at 0.8465s", or next to the "Stack:".

New related idea, maybe it should be another bug with UI/UX design:
It would be nice to show a small graph in the tooltip, with the different CPU samples. Especially when zoomed out, and the mouse points at a pixel column with more than 2 stack samples, Think of it as a magnifier revealing that there's more data in that small pixel column.
And since you'd show the multiple stack samples (say, as vertical bars), the one corresponding to the stack below could be highlighted.