Why PlotWidget does't scale in GridLayout cell?

Question

I put some widgets in a grid. But all the time there is a strange behavior of these widgets.

If you put one ImageView and PlotWidget near, ImageView cringe and PlotWidged stretched. I put near two ImageView and two PlotWidget to check - everything works fine. What's wrong?

import sys
import numpy as np
from PyQt5.QtWidgets import QWidget, QGridLayout, QPushButton, QApplication
import pyqtgraph as pg
from PIL import Image


class Example(QWidget):
    def __init__(self):
        super().__init__()
        self.initUI()

    def initUI(self):

        grid = QGridLayout()
        self.setLayout(grid)
        self.Imv = pg.ImageView()
        win = pg.PlotWidget()
        win.plot(np.random.random_sample(40)*10)
        btn1 = QPushButton("Start")
        btn2 = QPushButton('Forward/backward')
        btn3 = QPushButton('Filters')
        grid.addWidget(self.Imv, 0, 0, 1, 1)
        grid.addWidget(win, 0, 1, 1, 1)
        grid.addWidget(btn1, 1, 0, 1, 1)
        grid.addWidget(btn2, 2, 0, 1, 1)
        grid.addWidget(btn3, 3, 0, 1, 1)
        btn1.clicked.connect(self.cap)
        self.move(300, 150)
        self.setWindowTitle('Scentific camera view')
        self.show()

    def cap(self):
        global Imv
        img = np.array(Image.open('D:/Frames/image7.jpg'))
        self.Imv.setImage(img.T)

if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())

So it looks:

Two ImageView near - it's fine:

Answer 1

I would recomend to build your UI with QTDesigner and autogenerate the code. That makes things much easier. If you have to do it by hand try to set size policy to expanding or change the geometry to a fixed size:

win.setGeometry(QtCore.QRect(10, 20, 621, 181))
win.setSizePolicy(QtWidgets.QSizePolicy.Expanding)