Use a minimum height for legends of charts

The new legend will happily decrease its height to 0. To prevent this,
we need to make sure things have a useful minimum height.

faces/facepackages/barchart/contents/ui/FullRepresentation.qml

@@ -121,7 +121,7 @@ Faces.SensorFace {
             Layout.minimumHeight: root.formFactor === Faces.SensorFace.Horizontal
                 || root.formFactor === Faces.SensorFace.Vertical
                 ? implicitHeight
-                : -1
+                : Kirigami.Units.gridUnit
             visible: root.showLegend
             chart: compactRepresentation
             sourceModel: root.showLegend ? compactRepresentation.sensorsModel : null

faces/facepackages/linechart/contents/ui/FullRepresentation.qml

@@ -122,7 +122,7 @@ Faces.SensorFace {
             Layout.minimumHeight: root.formFactor === Faces.SensorFace.Horizontal
                 || root.formFactor === Faces.SensorFace.Vertical
                 ? implicitHeight
-                : -1
+                : Kirigami.Units.gridUnit
             visible: root.showLegend
             chart: compactRepresentation
             sourceModel: root.showLegend ? compactRepresentation.sensorsModel : null

faces/facepackages/piechart/contents/ui/FullRepresentation.qml

@@ -73,7 +73,7 @@ Faces.SensorFace {
             Layout.minimumHeight: root.formFactor === Faces.SensorFace.Horizontal
                 || root.formFactor === Faces.SensorFace.Vertical
                 ? implicitHeight
-                : -1
+                : Kirigami.Units.gridUnit
             visible: root.showLegend
             chart: compactRepresentation.chart
             sourceModel: root.showLegend ? compactRepresentation.sensorsModel : null