fix treemap rendering issue in firefox

[onesimus-wiafe]

This pull request fixes one of the Firefox issues in #2693, specifically issue #1983
I used string interpolation for the style on the rect element (for x and y) to introduce layout consistency by adding px to the d.x0 and d.y0 values. I realized that without this, Firefox ignored some of the values, hence the issue.

Pull Request Checklist

• Description which clearly states what problems the PR solves.
• Description contains a link to the Github Issue, and any relevent
  Discussions, this PR applies to.
• Include new tests that fail without this PR but passes with it.
• Include any relevant Documentation changes related to this change.
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[texodus]

Thanks for the PR!

Perspective does not currently support FireFox (See #2818) and its generally our policy not to merge fixes for unsupported platforms because they are difficult for correctness. However in this case, this fix is correct and trivial.