Improved Stats: Visitor count can be wrong

[rachelmcr]

Describe the bug

When I select the current month on the "This Year" tab, I expect the stats to match what I see on the "This Month" tab. However, the Visitors count doesn't match — the visitor count for "This Month" is too high.

It looks like the Visitors count for "This Month" may simply be adding up the total visitors for each day in the month. However, this is not accurate if the same visitor comes to the site on multiple days in the month.

To Reproduce
Steps to reproduce the behavior:

1. Log in to a store with WC Admin where at least one visitor has returned to the store on multiple days in the same month.
2. Enable the Improved Stats feature.
3. Select the "This Year" tab, select the current month, and note the number of visitors displayed.
4. Select the "This Month" tab and note that the number of visitors is different.

Screenshots

This Month tab (in January)	This Year tab (January selected)

Mobile Environment
Please include:

• Device: iPhone XS
• iOS version: 13.3
• WooCommerce iOS version: 3.5.0.0 TestFlight beta

[jaclync]

I noticed incorrect visitor count in "today" tab - it is showing 10, however I didn't visit the site at all today and "this week" shows 1 visitor count 😓 After some debugging, it seems like the "today" tab isn't getting the correct SiteVisitStats since both "today" and "this week" end on Sep 30 and one overrides the other in storage. That's why "today" tab is getting the visitor count on "this month" tab.

today	this week	this month

"This month" and "this year > this month" visitor count also don't match:

this month	this year > this month

[jaclync]

I'd recommend we add another attribute to SiteVisitStats that is unique for all 4 tabs (like timeRange enum). Also, we can use this opportunity to add an attribute for site ID. This requires schema migration, so holding off until another sprint.

[rachelmcr]

A user reported this issue for their store (internal ref: 3402266-zen). They noted the same scenario that I saw: When you are on the "This Year" tab and select a specific month, the visitor count for that month is off.

[reginabally]

A similar report from the Woo iOS app review:

We started an online business 2 mths ago This app was supposed to keep me up to date with traffic to the site etc as I’m not at the website My business partner is
I had to learn from our advertising company that the results this app was showing were nearly 600 unreported visits to the site Thank god the ppl running our ad campaign are on the ball because you clearly are not
I’m now looking at your reviews and wondering why my business partner choose your app at all The reviews are mostly bad

Date of review: Oct 13, 2020
iOS app version: 5.1

[jaclync]

Right now we save SiteVisitStats's granularity to the period unit like day/month for all 4 time ranges. However, there are 3 tabs with granularity day and their date could also be the same on certain days of the month. For example, today is Jan 25 (Monday) and the EOW (date for "This Week" tab) is Jan 31 (Sunday) which happens to be the same as the EOM (date for "This Month" tab). In the database, we now only have 3 SiteVisitStats entities:

while there should be 4 for the 4 tabs. Let's update SiteVisitStats's granularity to StatsTimeRangeV4 so that each SiteVisitStats is unique for all time ranges.