set options to make paintEvents faster; does not much for RAM usage (closes #104)

src/gui_qt/graphs/graphwindow.py

@@ -55,6 +55,11 @@ class QGraphWindow(QtWidgets.QGraphicsView, Ui_GraphWindow):
         self._external_items = []
         self.closable = True
 
+        # desperate attempts to improve memory usage during paintEvents
+        self.graphic.setAntialiasing(False)
+        self.plotItem.setDownsampling(auto=True)
+        self.plotItem.setClipToView(True)
+
         self._block = False
 
         self.log = [False, False]