Fix pandas SettingWithCopyWarning in Elexon forecast preprocessing

[SaitejaKommi]

Description

In the Elexon forecast data processing pipeline, assignment to start_time is performed on a DataFrame slice, which raises a pandas SettingWithCopyWarning.

This warning occurs when a transformation is applied to a view rather than a standalone DataFrame, which may lead to unpredictable behavior.

Expected Behavior

• No SettingWithCopyWarning should be emitted during Elexon preprocessing tests.
• Column assignments should operate on a safe DataFrame copy or use .loc.

Actual Behavior

• SettingWithCopyWarning is raised when assigning to start_time.

Scope

• elexon_plots.py only.

Suggested Fix

Avoid chained assignment by explicitly creating a copy of the DataFrame or using .loc before mutating columns.

Example approaches:

df = df.copy()
df["start_time"] = ...

or

df.loc[:, "start_time"] = ...

Validation

Run the following test to confirm the warning is resolved:

python -m pytest tests/test_elexon_plot.py -q

Verify:

• Tests pass successfully
• SettingWithCopyWarning no longer appears