Plot data from one another class

You have some serious problems with your code.

First of all, you are trying to create a new Plot instance, which doesn't make any sense, since one already exists and you need to access that.

Doing it like that also creates another important problem: you're just keeping a local reference to the graph widget, not its parent Plot instance; this results in that Plot being immediately destroyed, and since the graph widget is reparented to it (after being added to the layout), it gets immediately destroyed as well.

There are other issues too:

• you never add the button to the layout;
• you create unnecessary widgets and layouts;
• you're trying to overwrite parent of the MainWidget, but parent() is an existing function of all QObjects, and shall never be overwritten;

A better approach to do what you want is to connect to the button from the Plot instance, or, eventually use a custom signal:

class Data(QWidget):
    dataSignal = pyqtSignal(object, object)
    def __init__(self):
        QWidget.__init__(self)
        layout = QHBoxLayout(self)
        self.btn_off = QPushButton("Off")
        layout.addWidget(self.btn_off)
        self.btn_off.clicked.connect(self.ctrl)

    def ctrl(self):
        y = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
        x = [30, 32, 34, 32, 33, 31, 29, 32, 35, 45]
        self.dataSignal.emit(x, y)

class Plot(QWidget):
    def __init__(self):
        QWidget.__init__(self)
        layout = QVBoxLayout(self)
        self.graphWidget = pg.PlotWidget()
        layout.addWidget(self.graphWidget)
        widget2 = Data()
        layout.addWidget(widget2, stretch=4)

        widget2.dataSignal.connect(self.graphWidget.plot)


class MainWidget(QWidget):
    def __init__(self):
        QWidget.__init__(self)
        self.layout = QVBoxLayout()
        bottom_widget = Plot()
        self.layout.addWidget(bottom_widget, stretch=10)
        self.setLayout(self.layout)


class Window(QMainWindow):

    def __init__(self):
        QMainWindow.__init__(self)
        widget = MainWidget()
        self.setCentralWidget(widget)

import sys
app = QApplication(sys.argv)
win = Window()
win.show()
sys.exit(app.exec_())