@@ -2,8 +2,13 @@
 
 ## PlotPy Version 2.12.0 ##
 
+✨ New features:
+
+* **Min/max curve downsampling** — Curve downsampling now keeps, by default, the minimum and the maximum of each group of 2 × N points (N being the downsampling factor), instead of one point out of N: narrow peaks and the envelope of noisy signals are preserved, with about as many points as before. The new **Downsampling method** curve parameter (`CurveParam.dsamp_method`) restores the former method ("stride"). `CurveItem.get_dsamp_indices` returns the indices of the points kept, the new `minmax_downsampling_indices` function computes them, and the `CurveItem.frozen_downsampling` context keeps them while points are edited (closes [Issue #83](https://github.com/PlotPyStack/PlotPy/issues/83))
+
 🛠️ Bug fixes:
 
+* **Editing points of downsampled curves** — With downsampling enabled, moving a curve point modified another point of the curve data than the one shown, since the handle of a drawn point was used as an index into the full data
 * **Curve fitting dialog** — Running the automatic fit no longer raises `OverflowError: argument 1 overflowed` errors when a fitted value ends up far outside the range of its slider (e.g. with the Powell method on a fit range restricted to the tail of a peak). These errors left the slider of the parameter unresponsive, and the values of the following parameters were not displayed. Such values are now shown with a warning icon, whose tooltip gives the slider range, while the slider stays at its nearest end. Typing `nan` or `inf` in a parameter field now restores the previous value instead of raising errors, and the number of slider steps must be at least 2 in the parameter properties dialog (closes [Issue #76](https://github.com/PlotPyStack/PlotPy/issues/76))
 * **Cross-section panels with many hidden images** — X/Y cross-section panels kept one curve for every image ever displayed, even after it was hidden, so plots containing hundreds of hidden images became progressively slower to refresh. Curves are now only kept for visible images, and are recreated when a hidden image is shown again
 
 
@@ -4,6 +4,8 @@
 
 import math
 import sys
+from collections.abc import Generator
+from contextlib import contextmanager
 from typing import TYPE_CHECKING
 
 import numpy as np
@@ -39,6 +41,40 @@
 SELECTED_SYMBOL = SELECTED_SYMBOL_PARAM.build_symbol()
 
 
+def minmax_downsampling_indices(data: np.ndarray, factor: int) -> np.ndarray:
+    """Return the indices of the points kept by min/max downsampling
+
+    The data is split into groups of ``2 * factor`` points, and the indices of the
+    minimum and of the maximum of each group are kept, in increasing order, so that
+    peaks are preserved with about as many points as when keeping one point out of
+    ``factor``. The first and the last points are always kept. NaN values are
+    ignored, except in groups containing only NaN values, which keep one of them so
+    that the curve is still interrupted there.
+
+    Args:
+        data: Data to downsample (e.g. Y values of a curve)
+        factor: Downsampling factor
+
+    Returns:
+        Sorted indices of the points to keep
+    """
+    data = np.asarray(data, dtype=float)
+    size = data.size
+    if factor <= 1 or size <= 2:
+        return np.arange(size)
+    bucket = 2 * factor
+    nbuckets = -(-size // bucket)  # Last group may be incomplete
+    padded = np.full(nbuckets * bucket, np.nan)
+    padded[:size] = data
+    groups = padded.reshape(nbuckets, bucket)
+    isnan = np.isnan(groups)
+    imin = np.argmin(np.where(isnan, np.inf, groups), axis=1)
+    imax = np.argmax(np.where(isnan, -np.inf, groups), axis=1)
+    offsets = np.arange(nbuckets) * bucket
+    indices = np.concatenate(([0, size - 1], offsets + imin, offsets + imax))
+    return np.unique(indices[indices < size])
+
+
 def get_screen_pen(painter: QG.QPainter, pen: QG.QPen) -> QG.QPen:
     """Return the pen to draw a line on screen
 
@@ -222,6 +258,9 @@ def __init__(self, curveparam: CurveParam | None = None) -> None:
         self.immutable = True  # set to false to allow moving points around
         self._x = None
         self._y = None
+        self._dsamp_indices: np.ndarray | None = None
+        self._dsamp_size = 0
+        self._dsamp_frozen = False
         self.update_params()
 
     def _get_visible_axis_min(self, axis_id: int, axis_data: np.ndarray) -> float:
@@ -547,21 +586,69 @@ def update_data(self) -> None:
         if isinstance(self._x, np.ndarray) and isinstance(self._y, np.ndarray):
             self._setData(self._x, self._y)
 
-    def dsamp(self, data: np.ndarray) -> np.ndarray:
+    def dsamp(self, data: np.ndarray | None) -> np.ndarray | None:
         """Downsample data
 
         Args:
-            data: Data to downsample
+            data: Data to downsample (X, Y or error data of the curve)
 
         Returns:
             Downsampled data
         """
-        if self.param.use_dsamp and self.param.dsamp_factor > 1:
-            return data[:: self.param.dsamp_factor]
-        return data
+        if data is None or self._dsamp_indices is None:
+            return data
+        return data[self._dsamp_indices]
+
+    def get_dsamp_indices(self) -> np.ndarray | None:
+        """Return the indices of the points kept by downsampling
+
+        Returns:
+            Sorted indices of the points of the curve data that are drawn, or None if
+            the curve is not downsampled
+        """
+        return self._dsamp_indices
+
+    @contextmanager
+    def frozen_downsampling(self) -> Generator[None, None, None]:
+        """Keep the points kept by downsampling while the curve data is updated
+
+        This is useful when editing curve points: with min/max downsampling, a
+        point which is moved could otherwise be replaced by another point of its
+        group. The points kept by downsampling are computed again when the number
+        of points changes, or when the data is updated after this context.
+        """
+        self._dsamp_frozen = True
+        try:
+            yield
+        finally:
+            self._dsamp_frozen = False
+
+    def __update_dsamp_indices(self, values: np.ndarray) -> None:
+        """Update the indices of the points kept by downsampling
+
+        Args:
+            values: Curve values (Y data, or X data for vertical curves)
+        """
+        indices = self._dsamp_indices
+        if (
+            self._dsamp_frozen
+            and indices is not None
+            and self._dsamp_size == values.size
+        ):
+            return
+        self._dsamp_size = values.size
+        factor = self.param.dsamp_factor
+        if not self.param.use_dsamp or factor <= 1:
+            self._dsamp_indices = None
+        elif self.param.dsamp_method == "stride":
+            self._dsamp_indices = np.arange(0, values.size, factor)
+        else:
+            self._dsamp_indices = minmax_downsampling_indices(values, factor)
 
     def _setData(self, x: np.ndarray, y: np.ndarray) -> None:
         """Wrapper around QwtPlotCurve.setData() to handle downsampling"""
+        values = x if self.orientation() == QC.Qt.Vertical else y
+        self.__update_dsamp_indices(np.asarray(values))
         return super().setData(self.dsamp(x), self.dsamp(y))
 
     def set_data(self, x: np.ndarray, y: np.ndarray) -> None:
@@ -639,7 +726,12 @@ def hit_test(self, pos: QC.QPointF) -> tuple[float, float, bool, None]:
             p1x = plot.transform(ax, self._x[i + 1])
             p1y = plot.transform(ay, self._y[i + 1])
         distance = seg_dist(QC.QPointF(pos), QC.QPointF(p0x, p0y), QC.QPointF(p1x, p1y))
-        final_index = i // (int(not self.param.use_dsamp) or self.param.dsamp_factor)
+        indices = self._dsamp_indices
+        if indices is None:
+            final_index = int(i)
+        else:
+            # Index of the last point kept by downsampling at or before point `i`
+            final_index = max(int(np.searchsorted(indices, i, side="right")) - 1, 0)
         return distance, final_index, False, None
 
     def get_closest_coordinates(self, x: float, y: float) -> tuple[float, float]:
@@ -714,12 +806,19 @@ def move_local_point_to(
         """
         if self.immutable:
             return
+        indices = self._dsamp_indices
+        if indices is not None:
+            # The handle is the index of a point kept by downsampling
+            if handle < 0 or handle >= indices.size:
+                return
+            handle = int(indices[handle])
         if handle < 0 or handle > self._x.shape[0]:
             return
         x, y = canvas_to_axes(self, pos)
         self._x[handle] = x
         self._y[handle] = y
-        self._setData(self._x, self._y)
+        with self.frozen_downsampling():
+            self._setData(self._x, self._y)
         self.plot().replot()
 
     def move_local_shape(self, old_pos: QC.QPointF, new_pos: QC.QPointF) -> None:
 
@@ -355,15 +355,15 @@ msgstr "Police des valeurs"
 msgid "Scale"
 msgstr "Échelle"
 
-msgid "logarithmic"
-msgstr "logarithmique"
-
 msgid "linear"
 msgstr "linéaire"
 
 msgid "date/time"
 msgstr "date/heure"
 
+msgid "logarithmic"
+msgstr "logarithmique"
+
 msgid "Autoscale strategy"
 msgstr "Stratégie d'autoscale"
 
@@ -616,6 +616,22 @@ msgstr "Utiliser le sous-échantillonnage"
 msgid "Downsampling factor"
 msgstr "Facteur de sous-échantillonnage"
 
+msgid "Downsampling method"
+msgstr "Méthode de sous-échantillonnage"
+
+msgid "Min/max (peaks preserved)"
+msgstr "Min/max (pics préservés)"
+
+msgid "Stride (one point out of N)"
+msgstr "Pas (un point sur N)"
+
+msgid ""
+"Min/max: the minimum and the maximum of each group of 2 × N points are kept, so that peaks are preserved.\n"
+"Stride: one point out of N is kept."
+msgstr ""
+"Min/max : le minimum et le maximum de chaque groupe de 2 × N points sont conservés, de sorte que les pics sont préservés.\n"
+"Pas : un point sur N est conservé."
+
 msgid "Display"
 msgstr "Affichage"
 
@@ -648,12 +664,12 @@ msgstr "Classes"
 msgid "Number of bins"
 msgstr "Nombre de classes"
 
-msgid "Min"
-msgstr "Min"
-
 msgid "Minimum value"
 msgstr "Valeur minimale"
 
+msgid "Min"
+msgstr "Min"
+
 msgid "Maximum value"
 msgstr "Valeur maximale"
 
@@ -1707,3 +1723,4 @@ msgstr "Rotation et rognage"
 
 msgid "Show cropping rectangle"
 msgstr "Afficher le rectangle de rognage"
+
@@ -6,6 +6,7 @@
 
 from guidata.dataset import (
     BoolItem,
+    ChoiceItem,
     DataSet,
     FloatItem,
     GetAttrProp,
@@ -49,6 +50,19 @@ class CurveParam(DataSet):
     dsamp_factor = IntItem(_("Downsampling factor"), default=10, min=1).set_prop(
         "display", active=_use_dsamp_prop
     )
+    dsamp_method = ChoiceItem(
+        _("Downsampling method"),
+        (
+            ("minmax", _("Min/max (peaks preserved)")),
+            ("stride", _("Stride (one point out of N)")),
+        ),
+        default="minmax",
+        help=_(
+            "Min/max: the minimum and the maximum of each group of "
+            "2 × N points are kept, so that peaks are preserved.\n"
+            "Stride: one point out of N is kept."
+        ),
+    ).set_prop("display", active=_use_dsamp_prop)
 
     def update_param(self, curve: CurveItem) -> None:
         """Updates the parameters using values from a given CurveItem
 
@@ -63,8 +63,15 @@ def _make_curve_dsamp(dsamp_factor, use_dsamp):
     x = np.linspace(-10, 10, 1000)
     y = np.sin(x)
     curve = make.curve(x, y, dsamp_factor=dsamp_factor, use_dsamp=use_dsamp)
-    if use_dsamp:
+    if use_dsamp and dsamp_factor > 1:
+        # Min/max downsampling (default): about one point out of `dsamp_factor`
+        assert curve.dataSize() == curve.get_dsamp_indices().size
+        assert curve.dataSize() <= x[::dsamp_factor].size + 2
+        curve.param.dsamp_method = "stride"
+        curve.update_params()
         assert curve.dataSize() == x[::dsamp_factor].size
+    else:
+        assert curve.dataSize() == x.size
     return curve