Fix up constant splines

Zebedee Nichollsrequested to merge
fix-up-constant-splines into main

Description

Fixes up the constant splines. Once I saw these plotted, I realised the way I had done them was way more confusing than it needed to be.

Checklist

Please confirm that this pull request has done the following:

  • Tests added - [ ] Documentation added (where applicable)
  • Changelog item added to changelog/
Edited by Zebedee Nicholls

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