qml: mempool histogram color bar: show tooltips

to teach meanings of colours and positions in the bar

electrum/gui/qml/components/NetworkOverview.qml

@@ -91,6 +91,13 @@ Pane {
                                     Layout.fillWidth: true
                                     height: parent.height
                                     color: Qt.hsva(2/3-(2/3*(Math.log(Math.min(600, modelData[0]))/Math.log(600))), 0.8, 1, 1)
+                                    ToolTip.text: modelData[0] + " sat/vB around depth " + (modelData[2]/1000000).toFixed(2) + " MB"
+                                    ToolTip.visible: ma.containsMouse
+                                    MouseArea {
+                                        id: ma
+                                        anchors.fill: parent
+                                        hoverEnabled: true
+                                    }
                                 }
                             }
                         }

electrum/gui/qml/qenetwork.py

@@ -126,7 +126,11 @@ class QENetwork(QObject, QtEventListener):
                 break
             slot = min(item[1], bytes_limit-bytes_current)
             bytes_current += slot
-            capped_histogram.append([max(FEERATE_DEFAULT_RELAY/1000, item[0]), slot]) # clamped to [FEERATE_DEFAULT_RELAY/1000,inf]
+            capped_histogram.append([
+                max(FEERATE_DEFAULT_RELAY/1000, item[0]),  # clamped to [FEERATE_DEFAULT_RELAY/1000,inf[
+                slot,  # width of bucket
+                bytes_current,  # cumulative depth at far end of bucket
+            ])
 
         # add clamping attributes for the GUI
         self._fee_histogram = {