Improving the display of some graphics

[rdwebdesign]

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)
• I have Signed Off all commits. (git commit --signoff)

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What does this PR aim to accomplish?:

The previous PR (#1969) fixed what was proposed, but in some cases it might not display graphs for certain ranges.

Constraints for start and end dates also limited the scale for the X axis.

Especially on newly installed systems (with just a few hours of data), the graphs for "Last 7 Days", "Last 30 Days", "This Month", "This Year" and "All Time" would look exactly the same and with the same scale as the "Today" graphic, which can be confusing for beginners.

For the "Yesterday" and "Last Month" ranges, graphs would not even be displayed, if there was no data for those ranges.

How does this PR accomplish the above?:

With the new modifications, the graphics will work as expected, improving the user experience.

What documentation changes (if any) are needed to support this PR?:

none.

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-12-web-v5-9-and-core-v5-7-released/51795/1