AO3-7303 Fix flicker on hover for graph tooltip

[onefangg]

Pull Request Checklist

• Have you read "How to write the perfect pull request"?
• Have you read the contributing guidelines?
• Have you added tests for any changed functionality?
• No new assertions added because its a small CSS fix and steps to test google's charts will be very specific (or I lack the knowledge on how to generalise it)
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
• Do you have fewer than 5 pull requests already open? If not, please wait
  until they are reviewed and merged before creating new pull requests.

Issue

https://otwarchive.atlassian.net/browse/AO3-7303

Purpose

What does this PR do?

Fix flickering for tooltip statistic chart when direct mouseover on tooltip.

Testing Instructions

Testing instruction in the JIRA ticket

Credit

What name and pronouns should we use to credit you in the Archive of Our Own's Release Notes?

If you have a Jira account, please include the same name in the "Full name"
field on your Jira profile, so we can assign you the issues you're working on.

Please note that if you do not fill in this section, we will use your GitHub account name and
they/them pronouns.
Yi Fang (she / her)

[Bilka2]

Hi, Yi Fang!

Thank you so much for this pull request. Someone will be along to review it soon.

I've updated the Jira issue status to In Review so no one mistakenly opens a duplicate pull request. If you'd like the ability to comment on, assign, and transition issues in the future, you're welcome to create a Jira account! It makes things a bit easier for us on the organizational side if the Full Name on your Jira account either closely matches the name you'd like us to credit in the release notes or includes it in parentheses, e.g. "Nickname (CREDIT NAME)."

Once you've done that (or if you've already done it -- Jira has been unreliable about showing us new accounts in the admin panel lately), you can either reply here or send an email to otw-coders@transformativeworks.org with your account name and email address and we'll set up the permissions for you.

Thanks again for contributing! If you have any questions, you can contact us at the same email address listed above.

[onefangg]

thanks! my jira account name is "Yi Fang Chua", and my jira email is "onefangg@gmail.com"