history-graph: a "left boundary" in a tooltip may be confusing

[ildar170975]

Checklist

• [X] I have updated to the latest available Home Assistant version.
• [X] I have cleared the cache of my browser.
• [X] I have tried a different browser to see if it is related to my browser.

Describe the issue you are experiencing

Consider this card:

type: vertical-stack
cards:
  - type: history-graph
    entities:
      - sun.sun
    hours_to_show: 12
  - type: history-graph
    entities:
      - sun.sun
    hours_to_show: 24

Do not bother that the sun became "above_horizon" by a noon - it happened because I switched off test HA setup at night and switched on it at about a noon.

Check a tooltip for the 1st card:

It is said that the below_horizon state was between 08:38 and 11:39. But in fact 08:38 = current time 20:38 - hours_to_show, so this is not a real left boundary of this state.

Describe the behavior you expected

The left boundary must be a REAL left boundary.

Steps to reproduce the issue

as above

What version of Home Assistant Core has the issue?

2023.6.2

What was the last working version of Home Assistant Core?

No response

In which browser are you experiencing the issue with?

Chrome 114.0.5735.134

Which operating system are you using to run this browser?

No response

State of relevant entities

No response

Problem-relevant frontend configuration

No response

Javascript errors shown in your browser console/inspector

No response

Additional information

No response