data = np.random.random((64, 128)) * 2.0
x = np.arange(128)
y = np.arange(64)
fig, ax = plt.subplots(constrained_layout=True)
pcm = ax.pcolormesh(x, y, data)

#plt.colorbar() --> here it works fine
ax2 = ax.twinx()
fig.colorbar(pcm)
# --> here it gives wrong x values for the orange plot

ax2.plot(x, np.sin(x/10), color="orange", lw=4)

plt.show()

import sys
import numpy as np
from PyQt5 import QtWidgets

import matplotlib
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg
from matplotlib.backends.backend_qt5agg import NavigationToolbar2QT as NavigationToolbar
import matplotlib.pyplot as plt


matplotlib.use("Qt5Agg")


class MplCanvas(FigureCanvasQTAgg):
    def __init__(self, parent=None, width=5, height=4, dpi=100):
        self.fig = plt.figure(figsize=(width, height), dpi=dpi)
        self.axe = self.fig.add_subplot(1, 1, 1, label='good')
        super().__init__(self.fig)


class MainWindow(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__()

        v_layout = QtWidgets.QVBoxLayout()

        self.canvas = MplCanvas(self, width=5, height=4, dpi=100)
        self.axe = self.canvas.axe
        self.figure = self.canvas.fig
        
        # draw a scatter plot on self.axe
        np.random.seed(19680801)
        rx, ry = 3., 1.
        area = rx * ry * np.pi
        theta = np.arange(0, 2 * np.pi + 0.01, 0.1)
        verts = np.column_stack([rx / area * np.cos(theta), ry / area * np.sin(theta)])
        x, y, s, c = np.random.rand(4, 30)
        s *= 10 ** 2
        self.image = self.axe.scatter(x, y, s=s, c=c, marker=verts)

        # add a twinx: it fails the tight layout*********************
        x = np.arange(0, 1, 0.01)
        y = np.random.randn(len(x))
        self.ax2 = self.axe.twinx()
        self.ax2.plot(x, y, lw=2, linestyle='-', label='signal')
        # ***********************************************************

        toolbar = NavigationToolbar(self.canvas, self)
        v_layout.addWidget(toolbar)

        v_layout.addWidget(self.canvas)
        label = QtWidgets.QLabel('Show Color Bar')
        self.checkBox_colorBar = QtWidgets.QCheckBox(self)
        h_layout = QtWidgets.QHBoxLayout(self)
        h_layout.addWidget(label)
        h_layout.addWidget(self.checkBox_colorBar)

        v_layout.addLayout(h_layout)

        widget = QtWidgets.QWidget()
        widget.setLayout(v_layout)
        self.setCentralWidget(widget)

        self.checkBox_colorBar.toggled.connect(self.showColorBar)

    def showColorBar(self):

        color_bar = self.image.colorbar  # None if no color bar

        if self.checkBox_colorBar.isChecked():
            if color_bar is None:
                self.figure.colorbar(self.image)
        else:
            if color_bar is not None:
                color_bar.remove()
                del color_bar

        self.canvas.draw()


if __name__ == '__main__':

    app = QtWidgets.QApplication(sys.argv)

    window = MainWindow()
    mw = window
    mw.show()
    app.exec_()