Per-axis autoscale strategy (auto / fixed / none)

[PierreRaybaut]

Context

Today, the AutoScale action — triggered either by the toolbar button (DoAutoscaleTool) or by a middle mouse button click (AutoZoomHandler) — always recomputes the bounds of all enabled axes from the items' boundingRect(), applying a global margin (autoscale_margin_percent).

There is no way to configure this behavior on a per-axis basis. For example, one cannot ask for the X axis to be auto-scaled while pinning the Y axis to a user-defined vmin/vmax range, or vice versa.

Proposed enhancement

Introduce a per-axis autoscale strategy (4 axes: Y_LEFT, Y_RIGHT, X_BOTTOM, X_TOP) with three possible modes:

Strategy	Behavior
auto	Current behavior: compute bounds from items + margin.
fixed	Apply user-defined vmin / vmax bounds.
none	Leave the axis untouched on do_autoscale().

Default for every axis is auto → fully backward compatible.

User-facing impact

UI

• A new "Autoscale strategy" ChoiceItem is added to the axis-parameters dialog (already reachable via right-click on the axis scale → Axis parameters…).
• When the strategy is fixed, the existing Min / Max fields of AxisParam are reused as the bounds applied on the next AutoScale.
• The toolbar button and middle-click are unchanged on the surface: they now honor the strategy configured for each axis.

Programmatic API

• New methods on BasePlot:
  • set_axis_autoscale_strategy(axis_id, strategy, vmin=None, vmax=None)
  • get_axis_autoscale_strategy(axis_id) -> tuple[str, float | None, float | None]
• BasePlot.do_autoscale(replot=True, axis_id=None) signature is unchanged — it simply consults each axis's strategy before acting.
• An explicit do_autoscale(axis_id=...) call also honors the strategy (including none).

Edge cases

• Disabled axes: the existing if not self.axisEnabled(axis_id): continue guard at the start of do_autoscale keeps protecting inactive axes. The strategy is only consulted after this filter, so disabled axes have no side effects.
• Dialog: edit_axis_parameters is only invoked on an enabled axis, so the new ChoiceItem is only exposed for axes that are actually in use.