diff --git a/src/input/controller.cpp b/src/input/controller.cpp
index 05f4b37..9eaaa30 100644
--- a/src/input/controller.cpp
+++ b/src/input/controller.cpp
@@ -323,17 +323,27 @@ void Controller::on_key(const KeyEvent& e) {
   if (zooms) {
     const f64 notches = e.key == kKeyZoomIn ? kKeyZoomNotches : -kKeyZoomNotches;
     const f64 factor = zoom_factor(notches);
-    for (u32 i = 0; i < plot_->x_axis_count(); ++i) {
-      const Axis& a = plot_->x_axis(i);
-      const f64 lo = to_space(a.range.min, a.log);
-      const f64 hi = to_space(a.range.max, a.log);
-      zoom_x_axis(i, from_space(0.5 * (lo + hi), a.log), factor);
+    // Ctrl restricts the zoom to x, Alt to y; bare +/- zooms both. (Shift cannot be
+    // used: '+' is shift+'=' on common layouts, so key_from() folds them together and
+    // a bare zoom-in already arrives with Mod::Shift set.) Ctrl+Alt zooms neither,
+    // which is harmless and needs no special case.
+    const bool only_x = has_mod(e.mods, Mod::Ctrl);
+    const bool only_y = has_mod(e.mods, Mod::Alt);
+    if (!only_y) {
+      for (u32 i = 0; i < plot_->x_axis_count(); ++i) {
+        const Axis& a = plot_->x_axis(i);
+        const f64 lo = to_space(a.range.min, a.log);
+        const f64 hi = to_space(a.range.max, a.log);
+        zoom_x_axis(i, from_space(0.5 * (lo + hi), a.log), factor);
+      }
     }
-    for (u32 i = 0; i < plot_->y_axis_count(); ++i) {
-      const Axis& a = plot_->y_axis(i);
-      const f64 lo = to_space(a.range.min, a.log);
-      const f64 hi = to_space(a.range.max, a.log);
-      zoom_y_axis(i, from_space(0.5 * (lo + hi), a.log), factor);
+    if (!only_x) {
+      for (u32 i = 0; i < plot_->y_axis_count(); ++i) {
+        const Axis& a = plot_->y_axis(i);
+        const f64 lo = to_space(a.range.min, a.log);
+        const f64 hi = to_space(a.range.max, a.log);
+        zoom_y_axis(i, from_space(0.5 * (lo + hi), a.log), factor);
+      }
     }
   } else if (fits) {
     plot_->auto_fit_x();
diff --git a/tests/test_controller.cpp b/tests/test_controller.cpp
index 804afd8..275f6c5 100644
--- a/tests/test_controller.cpp
+++ b/tests/test_controller.cpp
@@ -263,6 +263,27 @@ PLOT_TEST(key_minus_zooms_out) {
   PLOT_CHECK(f.plot.x_range(f.ax).span() > 100.0);
 }
 
+PLOT_TEST(ctrl_plus_zooms_only_x) {
+  Fixture f;
+  f.ay = f.plot.add_y_axis({"v", {-1.0, 1.0}, false, plot::AxisPlace::Left, false});
+  const f64 x_before = f.plot.x_range(f.ax).span();
+  const f64 y_before = f.plot.y_range(f.ay).span();
+  f.controller.on_key({plot::input::kKeyZoomIn, kCtrl, true});
+  PLOT_CHECK(f.plot.x_range(f.ax).span() < x_before);
+  PLOT_CHECK_NEAR(f.plot.y_range(f.ay).span(), y_before, 1e-12);
+}
+
+PLOT_TEST(alt_plus_zooms_only_y) {
+  Fixture f;
+  f.ay = f.plot.add_y_axis({"v", {-1.0, 1.0}, false, plot::AxisPlace::Left, false});
+  constexpr plot::u32 kAlt = static_cast<plot::u32>(plot::input::Mod::Alt);
+  const f64 x_before = f.plot.x_range(f.ax).span();
+  const f64 y_before = f.plot.y_range(f.ay).span();
+  f.controller.on_key({plot::input::kKeyZoomIn, kAlt, true});
+  PLOT_CHECK_NEAR(f.plot.x_range(f.ax).span(), x_before, 1e-12);
+  PLOT_CHECK(f.plot.y_range(f.ay).span() < y_before);
+}
+
 /// A host can hand the controller a coordinate that is not a number: ImGui reports
 /// (-FLT_MAX, -FLT_MAX) for "no mouse position", and any host can produce a NaN out
 /// of a degenerate mapping of its own. The pan arithmetic runs through the axis