Set tooltip interaction mode for overtime graphs

[yubiuser]

What does this PR aim to accomplish?:

Fixes #2412.

When using the line graph style, multiple tooltips with partly doubled entries were shown for the same time slot. Setting the interaction mode to nearest (regarding the x-Axis) fixes the issue.

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[pralor-bot]

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

https://discourse.pi-hole.net/t/duplicate-data-on-dashboard/58721/29

[rdwebdesign]

Looks fine on my test.