fix: serialization of box/lasso select data for multiple subplots

CHANGELOG.md

@@ -21,6 +21,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Fix `convert_dash` mapping dotted lines (`linestyle=':'`) to invalid `'circle'` in Plotly, correctly converting them to `'dot'` and preserving custom dash patterns as pixel lists, [[#5719](https://github.com/plotly/plotly.py/pull/5719)] with thanks to @robertoffmoura for the contribution!
 - Fix `px.sunburst`, `px.treemap` and `px.icicle` listing sectors in a different order on every run when `path` is used with a Polars DataFrame; sectors now follow their order of first appearance for all dataframe backends [[#5766](https://github.com/plotly/plotly.py/pull/5766)], with thanks to @Irahan2 for the contribution!
 - Fix `mpl_to_plotly` conversion of matplotlib contour lines and line collections: close contour rings ending with `Z` codes, support dash styles, convert date x-axes, separate disjoint subpaths with `None` separators, hide line collection traces from the legend by default, and group consecutive same-style lines into single traces [[#5770](https://github.com/plotly/plotly.py/pull/5770)], with thanks to @robertoffmoura for the contribution!
+- Fix serialization of box/lasso select data for figures with multiple subplots [[#5710](https://github.com/plotly/plotly.py/pull/5710)], with thanks to @flpi-sun for the contribution!
 
 ## [7.1.0] - 2026-09-15
 

js/src/widget.ts

@@ -109,9 +109,7 @@ type Py2JsUpdateMsg = Py2JsMsg & {
 
 type Selector = {
   type: "box" | "lasso";
-  selector_state:
-    | { xrange: number[]; yrange: number[] }
-    | { xs: number[]; ys: number[] };
+  selector_state: Record<string, any>;
 };
 
 // Model
@@ -465,29 +463,14 @@ export class FigureModel {
        *  2 (secondary), and 4 (auxilary) were pressed during the event
        */
 
-      /**
-       * @typedef {Object} BoxSelectorState
-       * @property {Array.<Number>} xrange
-       *  Two element array containing the x-range of the box selection
-       * @property {Array.<Number>} yrange
-       *  Two element array containing the y-range of the box selection
-       */
-
-      /**
-       * @typedef {Object} LassoSelectorState
-       * @property {Array.<Number>} xs
-       *  Array of the x-coordinates of the lasso selection region
-       * @property {Array.<Number>} ys
-       *  Array of the y-coordinates of the lasso selection region
-       */
-
       /**
        * Object representing the state of the selection tool during a
        * plotly_select event
        * @typedef {Object} Selector
        * @property {String} type
        *  Selection type. One of: 'box', or 'lasso'
-       * @property {BoxSelectorState|LassoSelectorState} selector_state
+       * @property {Object.<String, Array>} selector_state
+       *  Selection geometry, keyed by plotly.js axis id
        */
 
       /**
@@ -1149,22 +1132,18 @@ export class FigureView {
     var selectorObject: Selector;
 
     if (data.hasOwnProperty("range")) {
-      // Box selection
+      // Box selection - preserve all subplot-specific axis keys (x, x2, x3, ... and y, y2, y3, ...)
+      var rangeData = data["range"];
       selectorObject = {
         type: "box",
-        selector_state: {
-          xrange: data["range"]["x"],
-          yrange: data["range"]["y"],
-        },
+        selector_state: rangeData,
       };
     } else if (data.hasOwnProperty("lassoPoints")) {
-      // Lasso selection
+      // Lasso selection - preserve all subplot-specific axis keys
+      var lassoData = data["lassoPoints"];
       selectorObject = {
         type: "lasso",
-        selector_state: {
-          xs: data["lassoPoints"]["x"],
-          ys: data["lassoPoints"]["y"],
-        },
+        selector_state: lassoData,
       };
     } else {
       selectorObject = null;

plotly/basewidget.py

@@ -9,6 +9,33 @@
 import anywidget
 
 
+def _selector_for_trace(selector_data, trace):
+    """
+    Return the selection as seen from one trace.
+
+    Plotly.js keys the payload by axis id, so a selection in the second
+    subplot arrives as {"x2": [...], "y2": [...]}. A trace drawn on another
+    subplot gets None, because that trace was not selected.
+    """
+    state = selector_data["selector_state"]
+    xaxis = getattr(trace, "xaxis", None) or "x"
+    yaxis = getattr(trace, "yaxis", None) or "y"
+
+    # "xrange"/"yrange" and "xs"/"ys" come from an older frontend bundle.
+    selector_type = selector_data["type"]
+    if selector_type == "box":
+        return BoxSelector(
+            xrange=state.get("xrange", state.get(xaxis)),
+            yrange=state.get("yrange", state.get(yaxis)),
+        )
+    elif selector_type == "lasso":
+        return LassoSelector(
+            xs=state.get("xs", state.get(xaxis)),
+            ys=state.get("ys", state.get(yaxis)),
+        )
+    raise ValueError("Unsupported selector type: %s" % selector_type)
+
+
 class BaseFigureWidget(BaseFigure, anywidget.AnyWidget):
     """
     Base class for FigureWidget. The FigureWidget class is code-generated as a
@@ -37,7 +64,7 @@ class BaseFigureWidget(BaseFigure, anywidget.AnyWidget):
     # appropriate _py2js_* property and then immediately assigning None to the
     # property.
     #
-    # See JSDoc comments in the FigureModel class in js/src/Figure.js for
+    # See JSDoc comments in the FigureModel class in js/src/widget.ts for
     # detailed descriptions of the messages.
     _py2js_addTraces = Dict(allow_none=True).tag(sync=True, **custom_serializers)
     _py2js_restyle = Dict(allow_none=True).tag(sync=True, **custom_serializers)
@@ -61,7 +88,7 @@ class BaseFigureWidget(BaseFigure, anywidget.AnyWidget):
     # responsible for setting the message property to None after retreiving
     # the message data.
     #
-    # See JSDoc comments in the FigureModel class in js/src/Figure.js for
+    # See JSDoc comments in the FigureModel class in js/src/widget.ts for
     # detailed descriptions of the messages.
     _js2py_traceDeltas = Dict(allow_none=True).tag(sync=True, **custom_serializers)
     _js2py_layoutDelta = Dict(allow_none=True).tag(sync=True, **custom_serializers)
@@ -651,18 +678,7 @@ def _handler_js2py_pointsCallback(self, change):
 
         # Build Selector Object
         # ---------------------
-        if callback_data.get("selector", None):
-            selector_data = callback_data["selector"]
-            selector_type = selector_data["type"]
-            selector_state = selector_data["selector_state"]
-            if selector_type == "box":
-                selector = BoxSelector(**selector_state)
-            elif selector_type == "lasso":
-                selector = LassoSelector(**selector_state)
-            else:
-                raise ValueError("Unsupported selector type: %s" % selector_type)
-        else:
-            selector = None
+        selector_data = callback_data.get("selector", None)
 
         # Build Input Device State Object
         # -------------------------------
@@ -710,6 +726,9 @@ def _handler_js2py_pointsCallback(self, change):
             elif event_type == "plotly_unhover":
                 trace._dispatch_on_unhover(points, state)
             elif event_type == "plotly_selected":
+                selector = (
+                    _selector_for_trace(selector_data, trace) if selector_data else None
+                )
                 trace._dispatch_on_selection(points, selector)
             elif event_type == "plotly_deselect":
                 trace._dispatch_on_deselect(points)

tests/test_core/test_figure_widget_backend/test_selection_callbacks.py

@@ -0,0 +1,96 @@
+"""
+Tests for the selection data that reaches an on_selection callback.
+
+Plotly.js keys box and lasso selection data by axis id, so a selection in
+the second subplot arrives as {"x2": [...], "y2": [...]}. The widget
+resolves that payload against each trace's own axes.
+"""
+
+import pytest
+
+import plotly.graph_objs as go
+from plotly.subplots import make_subplots
+
+try:
+    go.FigureWidget()
+except ImportError:
+    pytest.skip("FigureWidget is unavailable", allow_module_level=True)
+
+
+def two_subplots():
+    """Return a widget with one trace on x/y and another on x2/y2."""
+    fig = make_subplots(rows=1, cols=2)
+    fig.add_trace(go.Scatter(x=[1, 2, 3], y=[2, 1, 3]), row=1, col=1)
+    fig.add_trace(go.Scatter(x=[10, 11, 20, 21], y=[1, 2, 3, 4]), row=1, col=2)
+    return go.FigureWidget(fig)
+
+
+def select(widget, selector_state, selector_type="box"):
+    """
+    Fire a selection and return the selector each trace received.
+
+    The selection carries no points, because a box can cover a region of
+    the figure that holds no data. That is the case the ranges matter most.
+    """
+    selectors = []
+    for trace in widget.data:
+        trace.on_selection(lambda t, p, selector: selectors.append(selector))
+
+    widget._js2py_pointsCallback = {
+        "event_type": "plotly_selected",
+        "points": {"xs": [], "ys": [], "point_indexes": [], "trace_indexes": []},
+        "device_state": None,
+        "selector": {"type": selector_type, "selector_state": selector_state},
+    }
+    return selectors
+
+
+def test_box_selection_in_the_first_subplot():
+    first, second = select(two_subplots(), {"x": [1.1, 2.9], "y": [0.5, 3.5]})
+
+    assert first.xrange == [1.1, 2.9]
+    assert first.yrange == [0.5, 3.5]
+    # The other subplot was not selected, so it reports nothing.
+    assert second.xrange is None
+    assert second.yrange is None
+
+
+def test_box_selection_in_the_second_subplot():
+    first, second = select(two_subplots(), {"x2": [12.0, 19.0], "y2": [0.0, 5.0]})
+
+    assert second.xrange == [12.0, 19.0]
+    assert second.yrange == [0.0, 5.0]
+    assert first.xrange is None
+    assert first.yrange is None
+
+
+def test_box_selection_in_a_figure_without_subplots():
+    widget = go.FigureWidget(go.Scatter(x=[1, 2, 3], y=[3, 2, 1]))
+
+    (only,) = select(widget, {"x": [1.0, 2.0], "y": [1.5, 3.0]})
+
+    assert only.xrange == [1.0, 2.0]
+    assert only.yrange == [1.5, 3.0]
+
+
+def test_lasso_selection_in_the_second_subplot():
+    first, second = select(two_subplots(), {"x2": [1, 2, 3], "y2": [4, 5, 6]}, "lasso")
+
+    assert second.xs == [1, 2, 3]
+    assert second.ys == [4, 5, 6]
+    assert first.xs is None
+    assert first.ys is None
+
+
+def test_older_frontend_sends_xrange_and_yrange():
+    first, _ = select(two_subplots(), {"xrange": [1.0, 2.0], "yrange": [1.5, 3.0]})
+
+    assert first.xrange == [1.0, 2.0]
+    assert first.yrange == [1.5, 3.0]
+
+
+def test_older_frontend_sends_xs_and_ys():
+    first, _ = select(two_subplots(), {"xs": [1, 2, 3], "ys": [4, 5, 6]}, "lasso")
+
+    assert first.xs == [1, 2, 3]
+    assert first.ys == [4, 5, 6]