Add AI Coding Agent instructions for PythonQwt project

Author: PierreRaybaut

.github/copilot-instructions.md

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+# PythonQwt AI Coding Agent Instructions
+
+## Project Overview
+
+**PythonQwt** is a pure Python implementation of the Qwt C++ plotting library. It provides low-level Qt plotting widgets that form the foundation for higher-level libraries like PlotPy.
+
+### Technology Stack
+
+- **Python**: 3.9+
+- **Core**: NumPy (≥1.19), QtPy (≥1.9)
+- **GUI**: Qt via QtPy (PyQt5/PyQt6/PySide6)
+- **Testing**: pytest
+- **Linting**: Ruff, Pylint
+
+### Architecture
+
+```
+qwt/
+├── plot.py           # QwtPlot main widget
+├── plot_canvas.py    # Plot canvas
+├── plot_curve.py     # QwtPlotCurve
+├── plot_marker.py    # QwtPlotMarker
+├── plot_grid.py      # QwtPlotGrid
+├── scale_*.py        # Scale engine, map, division, drawing
+├── symbol.py         # QwtSymbol for markers
+├── legend.py         # QwtLegend
+├── text.py           # QwtText, QwtTextLabel
+├── graphic.py        # QwtGraphic
+└── tests/            # pytest suite
+```
+
+## Development Workflows
+
+### Running Commands
+
+Use batch scripts in `scripts/`:
+
+```powershell
+scripts\run_pytest.bat        # Run tests
+scripts\run_ruff.bat          # Format and lint
+scripts\run_pylint.bat        # Pylint checks
+scripts\run_coverage.bat      # Coverage report
+scripts\take_screenshots.bat  # Generate doc images
+```
+
+Or directly:
+
+```powershell
+python -m pytest qwt --ff
+python -m ruff format
+python -m ruff check --fix
+```
+
+### Running Test Launcher
+
+```powershell
+PythonQwt                           # GUI test launcher
+PythonQwt-tests --mode unattended   # Headless tests
+```
+
+Or from Python:
+
+```python
+from qwt import tests
+tests.run()
+```
+
+## Core Patterns
+
+### Basic Plot Creation
+
+```python
+import numpy as np
+from qtpy import QtWidgets as QW
+import qwt
+
+app = QW.QApplication([])
+
+# Create plot widget
+plot = qwt.QwtPlot("My Plot Title")
+plot.insertLegend(qwt.QwtLegend(), qwt.QwtPlot.BottomLegend)
+
+# Add curves
+x = np.linspace(0, 10, 100)
+qwt.QwtPlotCurve.make(x, np.sin(x), "Sine", plot,
+                       linecolor="blue", antialiased=True)
+
+# Add grid
+grid = qwt.QwtPlotGrid()
+grid.attach(plot)
+
+plot.resize(600, 400)
+plot.show()
+app.exec_()
+```
+
+### QwtPlotCurve Factory Method
+
+The `make` class method simplifies curve creation:
+
+```python
+curve = qwt.QwtPlotCurve.make(
+    x, y,                    # Data arrays
+    title="My Curve",        # Legend title
+    plot=plot,               # Parent plot (auto-attaches)
+    linecolor="red",         # Line color
+    linewidth=2,             # Line width
+    linestyle=Qt.DashLine,   # Qt line style
+    antialiased=True,        # Anti-aliasing
+    marker=qwt.QwtSymbol.Ellipse,  # Marker symbol
+    markersize=8,            # Marker size
+)
+```
+
+### Key Classes
+
+| Class | Purpose |
+|-------|---------|
+| `QwtPlot` | Main plot widget |
+| `QwtPlotCurve` | 2D curve item |
+| `QwtPlotMarker` | Point/line markers |
+| `QwtPlotGrid` | Grid lines |
+| `QwtLegend` | Legend widget |
+| `QwtSymbol` | Marker symbols |
+| `QwtScaleEngine` | Scale calculations |
+| `QwtScaleMap` | Scale transformations |
+| `QwtText` | Rich text labels |
+
+### Scale Configuration
+
+```python
+# Set axis titles
+plot.setAxisTitle(qwt.QwtPlot.xBottom, "Time (s)")
+plot.setAxisTitle(qwt.QwtPlot.yLeft, "Amplitude")
+
+# Set axis scale
+plot.setAxisScale(qwt.QwtPlot.xBottom, 0, 100)
+
+# Logarithmic scale
+plot.setAxisScaleEngine(qwt.QwtPlot.yLeft, qwt.QwtLogScaleEngine())
+```
+
+### Symbols and Markers
+
+```python
+# Create marker
+marker = qwt.QwtPlotMarker()
+marker.setSymbol(qwt.QwtSymbol(
+    qwt.QwtSymbol.Diamond,
+    QBrush(Qt.yellow),
+    QPen(Qt.red, 2),
+    QSize(10, 10)
+))
+marker.setValue(x_pos, y_pos)
+marker.attach(plot)
+```
+
+## Coding Conventions
+
+### Qt Imports
+
+Use QtPy for Qt binding abstraction:
+
+```python
+from qtpy.QtCore import Qt, QSize
+from qtpy.QtGui import QPen, QBrush, QColor
+from qtpy.QtWidgets import QWidget
+```
+
+### Docstrings
+
+Standard Python docstrings:
+
+```python
+def setData(self, x, y):
+    """Set curve data.
+
+    :param x: X coordinates (array-like)
+    :param y: Y coordinates (array-like)
+    """
+```
+
+## Key Files Reference
+
+| File | Purpose |
+|------|---------|
+| `qwt/plot.py` | QwtPlot implementation |
+| `qwt/plot_curve.py` | QwtPlotCurve with `make()` factory |
+| `qwt/scale_engine.py` | Linear/log scale engines |
+| `qwt/scale_map.py` | Scale transformations |
+| `qwt/symbol.py` | QwtSymbol definitions |
+| `qwt/tests/__init__.py` | Test launcher |
+
+## Limitations vs C++ Qwt
+
+The following are **not implemented** (PlotPy provides these):
+
+- `QwtPlotZoomer` - Use PlotPy's zoom tools
+- `QwtPicker` - Use PlotPy's interactive tools
+- `QwtPlotPicker` - Use PlotPy's selection tools
+
+Only essential plot items are implemented:
+- `QwtPlotItem` (base)
+- `QwtPlotCurve`
+- `QwtPlotMarker`
+- `QwtPlotGrid`
+- `QwtPlotSeriesItem`
+
+## Related Projects
+
+- **guidata**: Dataset/parameter framework (sibling)
+- **PlotPy**: High-level plotting using PythonQwt (downstream)