Bump plotters-backend from 0.3.6 to 0.3.7

Bumps [plotters-backend](https://github.com/plotters-rs/plotters) from 0.3.6 to 0.3.7.
- [Changelog](https://github.com/plotters-rs/plotters/blob/master/CHANGELOG.md)
- [Commits](plotters-rs/plotters@v0.3.6...v0.3.7)

---
updated-dependencies:
- dependency-name: plotters-backend
  dependency-type: indirect
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>