diff --git a/friture/Plot.qml b/friture/Plot.qml
index ac4ef74e..88e39daa 100644
--- a/friture/Plot.qml
+++ b/friture/Plot.qml
@@ -62,6 +62,8 @@ Rectangle {
             Layout.fillHeight: true
             Layout.fillWidth: true
 
+            scopedata: plot.scopedata
+
             vertical_axis: scopedata.vertical_axis
             horizontal_axis: scopedata.horizontal_axis
 
diff --git a/friture/PlotArea.qml b/friture/PlotArea.qml
index f49e6870..5ed1831f 100644
--- a/friture/PlotArea.qml
+++ b/friture/PlotArea.qml
@@ -8,6 +8,7 @@ Item {
 
     SystemPalette { id: systemPalette; colorGroup: SystemPalette.Active }
 
+    required property var scopedata
     required property Axis vertical_axis
     required property Axis horizontal_axis
 
@@ -31,6 +32,22 @@ Item {
         anchors.fill: parent
     }
 
+    Repeater {
+        id: annotationRepeater
+        anchors.fill: parent
+
+        model: scopePlotArea.scopedata.target_frequencies
+
+        Rectangle {
+            x: 0
+            y: parent.height * (1. - scopePlotArea.vertical_axis.coordinate_transform.toScreen(modelData) )
+            width: parent.width
+            height: 1
+            color: "lime"
+        }
+
+    }
+
     Rectangle {
         id: plotBorder
         anchors.fill: parent
@@ -87,5 +104,19 @@ Item {
         id: plotMouseArea
         anchors.fill: parent
         cursorShape: Qt.CrossCursor
+        acceptedButtons: Qt.LeftButton | Qt.RightButton
+
+        onClicked: {
+            if (mouse.button !== Qt.RightButton) { return; }
+            var freq = vertical_axis.coordinate_transform.toPlot((parent.height - mouse.y) / parent.height);
+            for (var existingFreq of scopedata.target_frequencies) {
+                var existingCoord = parent.height * (1 - scopePlotArea.vertical_axis.coordinate_transform.toScreen(existingFreq));
+                if (Math.abs(existingCoord - mouse.y) < 5) {
+                    scopedata.remove_target_frequency(existingFreq);
+                    return;
+                }
+            }
+            scopedata.add_target_frequency(freq);
+        }
     }
 }
diff --git a/friture/spectrogram_data.py b/friture/spectrogram_data.py
index eaa83917..994f7e52 100644
--- a/friture/spectrogram_data.py
+++ b/friture/spectrogram_data.py
@@ -17,9 +17,35 @@
 # You should have received a copy of the GNU General Public License
 # along with Friture.  If not, see <http://www.gnu.org/licenses/>.
 
+from PyQt5 import QtCore
+from PyQt5.QtCore import pyqtProperty, pyqtSlot
+
 from friture.scope_data import Scope_Data
 
 class Spectrogram_Data(Scope_Data):
+    target_frequencies_changed = QtCore.pyqtSignal()
 
     def __init__(self, parent=None):
         super().__init__(parent)
+        self._target_frequencies = []
+
+
+    @pyqtProperty('QVariantList', notify=target_frequencies_changed) # type: ignore
+    def target_frequencies(self):
+        return self._target_frequencies
+
+    @target_frequencies.setter
+    def target_frequencies(self, freqs):
+        if self._target_frequencies != freqs:
+            self._target_frequencies = freqs
+            self.target_frequencies_changed.emit()
+
+    @pyqtSlot(float)
+    def add_target_frequency(self, frequency):
+        if frequency not in self._target_frequencies:
+            self._target_frequencies.append(frequency)
+            self.target_frequencies_changed.emit()
+
+    @pyqtSlot(float)
+    def remove_target_frequency(self, frequency):
+        self.target_frequencies = [f for f in self._target_frequencies if f != frequency]