@@ -13,6 +13,7 @@
 * **Item tools and item list** — Updating the status of item-related tools and of the item list panel is no longer quadratic in the number of plot items
 * **Image creation** — Creating an image item no longer recomputes the colormap lookup table when the same colormap, inversion and transparency settings were already used: the table (more than 1000 colormap evaluations) is computed once and reused, which saves a few milliseconds per image
 * **Curves on high-DPI screens** — With a display scaling factor such as 125 % or 150 %, curves and error bars of width 1 were drawn wider than one screen pixel, which made their rendering up to several hundred times slower on noisy signals (about 800 ms instead of 3 ms for a 2000-point curve at 150 %). Curves and error bars of width 1 or less are now drawn one screen pixel wide on screen; printing and exporting are not affected. The new `curve/cosmetic_hairline` option of the `plot` configuration section restores the previous behavior (closes [Issue #80](https://github.com/PlotPyStack/PlotPy/issues/80))
+* **Thick curves** — Curves wider than one screen pixel are now drawn on screen as several one-pixel strokes, slightly offset from each other, which looks almost the same as before but is 30 to 50 times faster on noisy curves: a 2000-point curve of width 2 takes about 10 ms instead of 370 ms to draw with antialiasing. Dash patterns are scaled as before. Printing and exporting, curves with a translucent color and curves wider than 8 screen pixels still use the original pen. The new `curve/fast_thick_lines` option of the `plot` configuration section restores the previous behavior (closes [Issue #82](https://github.com/PlotPyStack/PlotPy/issues/82))
 
 ⚙️ Dependencies:
 
 
@@ -50,6 +50,8 @@ def get_plotpy_defaults() -> dict[str, int | float | str | bool]:
             "antialiasing": False,
             # Draw thin curves (width <= 1) one device pixel wide on screen
             "curve/cosmetic_hairline": True,
+            # Draw thick curves as several one-pixel strokes on screen
+            "curve/fast_thick_lines": True,
             "title/font/size": 12,
             "title/font/bold": False,
             "selected_curve_symbol/marker": "Rect",
 
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import math
 import sys
 from typing import TYPE_CHECKING
 
@@ -11,6 +12,7 @@
 from qtpy import QtCore as QC
 from qtpy import QtGui as QG
 from qwt import QwtPlotCurve
+from qwt.plot_curve import series_to_polyline
 
 from plotpy.config import CONF, _
 from plotpy.coords import canvas_to_axes
@@ -70,6 +72,68 @@ def get_screen_pen(painter: QG.QPainter, pen: QG.QPen) -> QG.QPen:
     return pen
 
 
+#: Maximum width, in device pixels, of the lines drawn as several one-pixel strokes
+#: (see :py:func:`get_thick_line_strokes`): wider lines are drawn with their pen
+THICK_LINE_MAX_STROKES_WIDTH = 8.0
+
+
+def get_thick_line_strokes(
+    painter: QG.QPainter, pen: QG.QPen
+) -> tuple[QG.QPen, list[tuple[float, float]]] | None:
+    """Return the strokes drawing a thick line quickly on screen
+
+    Qt's raster engine draws pens wider than one device pixel much more slowly
+    than one-pixel pens (up to a hundred times slower for noisy curves, with
+    antialiasing). When painting on screen (raster engine, without scaling
+    transform), an opaque solid line wider than one device pixel may instead be
+    drawn as several one-device-pixel strokes, offset horizontally and vertically
+    by at most one device pixel from each other so as to cover the line width,
+    whatever the line direction. Dash patterns are scaled by the line width, as
+    for the original pen.
+
+    The line is drawn with its pen when printing or exporting (other paint engines
+    or scaling transform), for translucent colors (whose overlapping strokes would
+    accumulate), for lines wider than :py:data:`THICK_LINE_MAX_STROKES_WIDTH`
+    device pixels, or when the ``curve/fast_thick_lines`` option of the ``plot``
+    configuration section is disabled.
+
+    Args:
+        painter: Painter
+        pen: Pen to draw with
+
+    Returns:
+        A tuple (stroke pen, stroke offsets in logical coordinates), or None if
+        the line must be drawn with the pen itself
+    """
+    if (
+        pen.style() == QC.Qt.NoPen
+        or pen.isCosmetic()
+        or pen.brush().style() != QC.Qt.SolidPattern
+        or pen.color().alpha() < 255
+        or painter.paintEngine().type() != QG.QPaintEngine.Raster
+        or painter.worldTransform().isScaling()
+    ):
+        return None
+    ratio = painter.device().devicePixelRatioF()
+    width = pen.widthF() * ratio
+    if (
+        width <= 1.0
+        or width > THICK_LINE_MAX_STROKES_WIDTH
+        or not CONF.get("plot", "curve/fast_thick_lines", True)
+    ):
+        return None
+    stroke_pen = QG.QPen(pen)
+    stroke_pen.setWidthF(1.0)
+    stroke_pen.setCosmetic(True)
+    if pen.style() != QC.Qt.SolidLine:
+        stroke_pen.setDashPattern([length * width for length in pen.dashPattern()])
+    extra = width - 1.0
+    shifts = np.linspace(-extra / 2, extra / 2, math.ceil(extra) + 1) / ratio
+    offsets = [(float(shift), 0.0) for shift in shifts]
+    offsets += [(0.0, float(shift)) for shift in shifts if shift != 0.0]
+    return stroke_pen, offsets
+
+
 def seg_dist(P: QC.QPointF, P0: QC.QPointF, P1: QC.QPointF) -> float:
     """Compute distance between point P and segment (P0, P1)
 
@@ -228,6 +292,47 @@ def drawCurve(
             painter.setPen(screen_pen)
         super().drawCurve(painter, style, xMap, yMap, canvasRect, from_, to)
 
+    def drawLines(
+        self,
+        painter: QG.QPainter,
+        xMap: QwtScaleMap,
+        yMap: QwtScaleMap,
+        canvasRect: QC.QRectF,
+        from_: int,
+        to: int,
+    ) -> None:
+        """Draw lines
+
+        Reimplemented to draw thick lines quickly on screen, as several one-pixel
+        strokes (see :py:func:`get_thick_line_strokes`).
+
+        Args:
+            painter: Painter
+            xMap: X axis scale map
+            yMap: Y axis scale map
+            canvasRect: Contents rectangle of the canvas
+            from_: Index of the first point to be painted
+            to: Index of the last point to be painted
+        """
+        strokes = get_thick_line_strokes(painter, painter.pen())
+        if strokes is None:
+            super().drawLines(painter, xMap, yMap, canvasRect, from_, to)
+            return
+        if from_ > to:
+            return
+        stroke_pen, offsets = strokes
+        polyline = series_to_polyline(xMap, yMap, self.data(), from_, to)
+        painter.save()
+        painter.setPen(stroke_pen)
+        for dx, dy in offsets:
+            painter.translate(dx, dy)
+            painter.drawPolyline(polyline)
+            painter.translate(-dx, -dy)
+        painter.restore()
+        brush = self.brush()
+        if brush.style() != QC.Qt.NoBrush and brush.color().alpha() > 0:
+            self.fillCurve(painter, xMap, yMap, canvasRect, polyline)
+
     def types(self) -> tuple[type[IItemType], ...]:
         """Returns a group or category for this item.
         This should be a tuple of class objects inheriting from IItemType
 
@@ -0,0 +1,168 @@
+# -*- coding: utf-8 -*-
+#
+# Licensed under the terms of the BSD 3-Clause
+# (see plotpy/LICENSE for details)
+
+"""Test the fast drawing of thick curves as several one-pixel strokes"""
+
+from __future__ import annotations
+
+import numpy as np
+import pytest
+from guidata.qthelpers import qt_app_context
+from qtpy import QtCore as QC
+from qtpy import QtGui as QG
+from qwt import QwtScaleMap
+
+from plotpy.builder import make
+from plotpy.config import CONF
+from plotpy.items.curve import base as curve_base
+from plotpy.items.curve.base import (
+    THICK_LINE_MAX_STROKES_WIDTH,
+    get_thick_line_strokes,
+)
+
+WIDTH, HEIGHT = 200, 120
+
+
+def _image(ratio: float = 1.0) -> QG.QImage:
+    """Return a white image with the given device pixel ratio"""
+    image = QG.QImage(int(WIDTH * ratio), int(HEIGHT * ratio), QG.QImage.Format_ARGB32)
+    image.setDevicePixelRatio(ratio)
+    image.fill(QC.Qt.white)
+    return image
+
+
+def _set_option(monkeypatch: pytest.MonkeyPatch, enabled: bool) -> None:
+    """Set the ``curve/fast_thick_lines`` option"""
+    original_get = CONF.get
+
+    def get(section, option, *args, **kwargs):
+        if (section, option) == ("plot", "curve/fast_thick_lines"):
+            return enabled
+        return original_get(section, option, *args, **kwargs)
+
+    monkeypatch.setattr(curve_base.CONF, "get", get)
+
+
+def _strokes(image: QG.QImage, pen: QG.QPen, scale: float = 1.0):
+    """Return the strokes of a pen for a painter on an image"""
+    painter = QG.QPainter(image)
+    if scale != 1.0:
+        painter.scale(scale, scale)
+    strokes = get_thick_line_strokes(painter, pen)
+    painter.end()
+    return strokes
+
+
+def _render(
+    width: float, color: str = "#000000", style: str = "-", ratio: float = 1.0
+) -> np.ndarray:
+    """Render a noisy sine curve and return the image darkness (0 to 1)"""
+    rng = np.random.default_rng(0)
+    x = np.linspace(0.0, 1.0, 400)
+    y = 0.5 + 0.3 * np.sin(12 * x) + 0.03 * rng.standard_normal(x.size)
+    curve = make.curve(x, y, color=color, linewidth=width, linestyle=style)
+    xmap, ymap = QwtScaleMap(), QwtScaleMap()
+    xmap.setPaintInterval(0, WIDTH)
+    xmap.setScaleInterval(0.0, 1.0)
+    ymap.setPaintInterval(HEIGHT, 0)
+    ymap.setScaleInterval(0.0, 1.0)
+    image = _image(ratio)
+    painter = QG.QPainter(image)
+    painter.setRenderHint(QG.QPainter.Antialiasing, True)
+    curve.drawSeries(painter, xmap, ymap, QC.QRectF(0, 0, WIDTH, HEIGHT), 0, -1)
+    painter.end()
+    gray = image.convertToFormat(QG.QImage.Format_Grayscale8)
+    data = np.array(
+        [
+            [gray.pixelColor(col, row).value() for col in range(gray.width())]
+            for row in range(gray.height())
+        ],
+        dtype=float,
+    )
+    return 1.0 - data / 255.0
+
+
+def test_thick_line_strokes_offsets() -> None:
+    """Strokes cover the line width, horizontally and vertically"""
+    with qt_app_context(exec_loop=False):
+        pen = QG.QPen(QG.QColor("#000000"), 2.0)
+        stroke_pen, offsets = _strokes(_image(), pen)
+        assert stroke_pen.isCosmetic() and stroke_pen.widthF() == 1.0
+        assert sorted(offsets) == [(-0.5, 0.0), (0.0, -0.5), (0.0, 0.5), (0.5, 0.0)]
+        # Width of 3 device pixels with a device pixel ratio of 1.5:
+        _stroke_pen, offsets = _strokes(_image(1.5), pen)
+        assert len(offsets) == 5
+        shifts = sorted(dx * 1.5 for dx, _dy in offsets if _dy == 0.0)
+        assert shifts == pytest.approx([-1.0, 0.0, 1.0])
+        assert not pen.isCosmetic()  # The original pen is not modified
+
+
+def test_thick_line_strokes_dash_pattern() -> None:
+    """Dash patterns of strokes are scaled by the line width"""
+    with qt_app_context(exec_loop=False):
+        pen = QG.QPen(QG.QColor("#000000"), 2.0, QC.Qt.DashLine)
+        stroke_pen, _offsets = _strokes(_image(), pen)
+        assert list(stroke_pen.dashPattern()) == [
+            2.0 * length for length in pen.dashPattern()
+        ]
+
+
+@pytest.mark.parametrize(
+    "pen_args, scale",
+    [
+        (("#000000", 1.0), 1.0),  # Thin line
+        (("#80000000", 2.0), 1.0),  # Translucent color
+        (("#000000", THICK_LINE_MAX_STROKES_WIDTH + 1.0), 1.0),  # Very wide line
+        (("#000000", 2.0), 3.0),  # Scaled painter (printing, exporting)
+    ],
+)
+def test_thick_line_pen_kept(pen_args: tuple[str, float], scale: float) -> None:
+    """Lines are drawn with their pen when strokes do not apply"""
+    with qt_app_context(exec_loop=False):
+        color, width = pen_args
+        pen = QG.QPen(QG.QColor(color), width)
+        assert _strokes(_image(), pen, scale) is None
+
+
+def test_thick_line_other_engine() -> None:
+    """Lines are drawn with their pen on other paint devices"""
+    with qt_app_context(exec_loop=False):
+        picture = QG.QPicture()
+        painter = QG.QPainter(picture)
+        assert get_thick_line_strokes(painter, QG.QPen(QG.QColor("red"), 2.0)) is None
+        painter.end()
+
+
+def test_thick_line_option(monkeypatch: pytest.MonkeyPatch) -> None:
+    """The ``curve/fast_thick_lines`` option disables strokes"""
+    _set_option(monkeypatch, False)
+    with qt_app_context(exec_loop=False):
+        assert _strokes(_image(), QG.QPen(QG.QColor("red"), 2.0)) is None
+
+
+@pytest.mark.parametrize("ratio", [1.0, 1.5])
+@pytest.mark.parametrize("width", [1.5, 2.0, 3.0])
+def test_thick_line_rendering(
+    monkeypatch: pytest.MonkeyPatch, width: float, ratio: float
+) -> None:
+    """Strokes look like the line drawn with its pen"""
+    with qt_app_context(exec_loop=False):
+        _set_option(monkeypatch, True)
+        strokes = _render(width, ratio=ratio)
+        _set_option(monkeypatch, False)
+        reference = _render(width, ratio=ratio)
+        # Same amount of ink, at the same place:
+        assert strokes.sum() == pytest.approx(reference.sum(), rel=0.1)
+        assert np.abs(strokes - reference).sum() < 0.2 * reference.sum()
+        assert not np.array_equal(strokes, reference)  # Strokes were used
+
+
+def test_translucent_thick_line_rendering(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Translucent thick lines are drawn with their pen"""
+    with qt_app_context(exec_loop=False):
+        _set_option(monkeypatch, True)
+        image = _render(2.0, color="#80ff0000")
+        _set_option(monkeypatch, False)
+        np.testing.assert_array_equal(image, _render(2.0, color="#80ff0000"))