[Feature]: Add plotting for SIST results

[harryswift01]

Feature Request

Problem / Motivation

SIST does not currently provide maintained plotting support, so users must write their own scripts or manually extract values.

Proposed Solution

Review any historical plotting code and implement a small, supported set of plots using public result objects and saved JSON files.

Proposed Work

• Review historical plotting scripts.
• Agree the initial plot set with scientific users.
• Define the required values, units, and labels.
• Implement plots from result objects and saved files.
• Return normal Matplotlib figure and axes objects.
• Handle missing or incomplete data clearly.
• Add tests, documentation, and notebook examples.
• Decide whether Matplotlib is a required or optional dependency.

Done When

• The supported plot set has been agreed.
• Standard plots work with saved results.
• No calculation rerun is required.
• Axes and units are labelled correctly.
• Missing data produces a clear warning or error.
• The plotting API is tested and demonstrated.