[henrikjacobsenfys]

Closes #205.

Diffusion models are usually read off a plot of the widths against Q², where Fickian diffusion (Γ = D·Q²) is a straight line. Both parameter plots now take an x_axis argument:

analysis.plot_parameters(names='Lorentzian width', x_axis='Q^2')
parameter_analysis.plot(x_axis='Q^2')

The default is 'Q', so existing calls are unchanged.

How it works

• Nothing is recomputed. Data and model values at each Q stay as they are; only the coordinate they are plotted against changes. A Q² coordinate is added to the Dataset, and plopp is told to use it with coords=[x_axis]. The axis label and unit follow from scipp (Q^2 [1/Å^2]).
• The logic is shared in easydynamics.utils.plotting:
  • Q_AXIS_TRANSFORMS maps each axis name to a function of Q.
  • add_x_axis_coord(data, x_axis) checks the name and returns a copy of the Dataset with that coordinate added.
• Adding another axis, such as '1/Q', is one entry in Q_AXIS_TRANSFORMS. Both plot methods pick it up automatically.

Tutorial 1 now plots the Lorentzian width against Q² before fitting the Brownian diffusion model.

Notes for review

• x_axis is placed after names, so positional calls that already passed names still work.
• test_plot_parameters in test_analysis.py used placeholder objects instead of data. It now uses a real sc.Dataset, since the method reads its Q coordinate.
• plot_parameters' docstring said kwargs go to plopp.slicer; it now says plopp.plot, which is where they go.
• Checked with an unmocked ParameterAnalysis.plot of a D·Q² model: data points and model line both land at x = Q² and form a straight line.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.15%. Comparing base (762b081) to head (58814d1).
⚠️ Report is 6 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #271   +/-   ##
========================================
  Coverage    98.15%   98.15%           
========================================
  Files           60       60           
  Lines         6066     6074    +8     
  Branches      1022     1023    +1     
========================================
+ Hits          5954     5962    +8     
  Misses          58       58           
  Partials        54       54           

Flag	Coverage Δ
unittests	98.15% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
src/easydynamics/analysis/analysis.py	97.68% <100.00%> (+<0.01%)	⬆️
src/easydynamics/analysis/parameter_analysis.py	99.27% <100.00%> (+<0.01%)	⬆️
src/easydynamics/utils/plotting.py	89.74% <100.00%> (+1.86%)	⬆️

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