title: PyQtGraph doesn't resize
body: <p>I'm trying to make simplest Qt app to visualize some data using pyqtgraph lib. I've created one-window app using Qt Designer, placed there Graphics View widget, promoted it to pygtgraph. In my app (written in python) I create test data set and plot it. This works (graph is displayed correctly) but the graph doesn't re-size with the window. So then, in Qt Designer I set layout of main form to "Lay Out in a grid" and in preview it works fine ("Graphics View" widget resizing with main window). But when I run my app then the plot appears very small, like 5x20 pixels and is not re-sizable.</p>
  
  <p>My app:</p>
  
  <pre><code>class AppWindow(QtWidgets.QMainWindow, StartForm.Ui_StartForm):
      def __init__(self):
          super(AppWindow, self).__init__()
          self.setupUi(self)
  
          line1 = ([1, 3, 2, 4, 6, 5, 3])
          pl = self.graphicsView.plot(line1)  # graphicsView is Graphics View widget from Qt Designer
  
  app = QApplication(sys.argv)
  w = AppWindow()
  w.show()
  sys.exit(app.exec_())
  </code></pre>
  
  <p>generated code by Qt Designer:</p>
  
  <pre><code>from PyQt5 import QtCore, QtGui, QtWidgets
  
  
  class Ui_StartForm(object):
      def setupUi(self, StartForm):
          StartForm.setObjectName("StartForm")
          StartForm.resize(1609, 1062)
          self.graphicsView = PlotWidget(StartForm)
          self.graphicsView.setGeometry(QtCore.QRect(11, 11, 1261, 931))
          self.graphicsView.setObjectName("graphicsView")
  
          self.retranslateUi(StartForm)
          QtCore.QMetaObject.connectSlotsByName(StartForm)
  
      def retranslateUi(self, StartForm):
          _translate = QtCore.QCoreApplication.translate
          StartForm.setWindowTitle(_translate("StartForm", "Form"))
  
  
  from pyqtgraph import PlotWidget
  </code></pre>
  
  <p>I've also tried to create pyqtgraph plot from my python app and then embed it into empty layout generated by Qt Designer but the result is the same - plot is not resizable. Seems like it doesn't inherit some properties from main form.</p>
  
  <p>So the question is - why my graph appears very small (doesn't expand to full window as it does in Qt Designer pre-view) and is not re-sizable? How to fix this? </p>