Fix NOAAIndices plot limits

[dstansby]

Fixes #5357. Because there are lots of NaNs in the files, and pandas will set the x-limits including these, the solution is to manually set the x-limits ignoring the NaNs. I also did a bit of refactoring to make the code nicer.

[sunpy-backport]

The backport to 3.0 failed:

Commits ["82e3fb08443a01d3d55f5645bf62367578d10a6b","5ee0ddd52e31f3057dcac6afa240ef61adb1c167"] could not be cherry-picked on top of 3.0

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport 3.0
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 82e3fb08443a01d3d55f5645bf62367578d10a6b 5ee0ddd52e31f3057dcac6afa240ef61adb1c167
# Create a new branch with these backported commits.
git checkout -b backport-5496-to-3.0
# Push it to GitHub.
git push --set-upstream origin backport-5496-to-3.0
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is 3.0 and the compare/head branch is backport-5496-to-3.0.