Updated mpanel title sizing to allow it to extend across the whole chart

Describe your changes

[ ] I have written tests covering functions I have added or changed.
[ ] I have run tests and they all pass.
[ ] I have ensured all new functions will show up in the _pkgdown.yml file.
[ ] I have updated DESCRIPTION with any new package dependencies.
[ ] I have updated NEWS.md with a brief description of my changes.
[ ] I have updated the package documentation with devtools::document().
[ ] I have updated the package website with pkgdown::build_site_github_pages().