Построения графика pyqtgraph в PyQt5

Я увидел вашу задачу так:

import sys
from datetime import datetime                                   # +++
from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.Qt import *
from pyqtgraph import PlotWidget, plot                          # +++
import pyqtgraph as pg                                          # +++


#from test_ui import Ui_MainWindow
class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        if not MainWindow.objectName():
            MainWindow.setObjectName(u"MainWindow")
        MainWindow.resize(320, 240)
        self.centralwidget = QWidget(MainWindow)
        self.centralwidget.setObjectName(u"centralwidget")
        self.gridLayout = QGridLayout(self.centralwidget)
        self.gridLayout.setObjectName(u"gridLayout")
        self.tabWidget = QTabWidget(self.centralwidget)
        self.tabWidget.setObjectName(u"tabWidget")
        self.tab = QWidget()
        self.tab.setObjectName(u"tab")
        self.gridLayout_3 = QGridLayout(self.tab)
        self.gridLayout_3.setObjectName(u"gridLayout_3")
        self.tableWidget = QTableWidget(self.tab)
        if (self.tableWidget.columnCount() < 2):
            self.tableWidget.setColumnCount(2)
        __qtablewidgetitem = QTableWidgetItem()
        self.tableWidget.setHorizontalHeaderItem(0, __qtablewidgetitem)
        __qtablewidgetitem1 = QTableWidgetItem()
        self.tableWidget.setHorizontalHeaderItem(1, __qtablewidgetitem1)
        if (self.tableWidget.rowCount() < 3):
            self.tableWidget.setRowCount(3)
        __qtablewidgetitem2 = QTableWidgetItem()
        self.tableWidget.setVerticalHeaderItem(0, __qtablewidgetitem2)
        __qtablewidgetitem3 = QTableWidgetItem()
        self.tableWidget.setVerticalHeaderItem(1, __qtablewidgetitem3)
        __qtablewidgetitem4 = QTableWidgetItem()
        self.tableWidget.setVerticalHeaderItem(2, __qtablewidgetitem4)
        self.tableWidget.setObjectName(u"tableWidget")
        self.tableWidget.horizontalHeader().setDefaultSectionSize(130)
        self.gridLayout_3.addWidget(self.tableWidget, 0, 0, 1, 1)

        self.pushButton = QPushButton(self.tab)
        self.pushButton.setObjectName(u"pushButton")
        self.gridLayout_3.addWidget(self.pushButton, 1, 0, 1, 1)

        self.tabWidget.addTab(self.tab, "")
        self.tab_2 = QWidget()
        self.tab_2.setObjectName(u"tab_2")
        self.gridLayout_2 = QGridLayout(self.tab_2)
        self.gridLayout_2.setObjectName(u"gridLayout_2")
        self.widget = PlotWidget(self.tab_2)
        self.widget.setObjectName(u"widget")
        self.gridLayout_2.addWidget(self.widget, 0, 0, 1, 1)

        self.tabWidget.addTab(self.tab_2, "")
        self.gridLayout.addWidget(self.tabWidget, 0, 0, 1, 1)

        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QMenuBar(MainWindow)
        self.menubar.setObjectName(u"menubar")
        self.menubar.setGeometry(QRect(0, 0, 320, 21))
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QStatusBar(MainWindow)
        self.statusbar.setObjectName(u"statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        self.tabWidget.setCurrentIndex(0)
        QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        MainWindow.setWindowTitle(QCoreApplication.translate("MainWindow", u"MainWindow", None))
        ___qtablewidgetitem = self.tableWidget.horizontalHeaderItem(0)
        ___qtablewidgetitem.setText(QCoreApplication.translate("MainWindow", u"1", None));
        ___qtablewidgetitem1 = self.tableWidget.horizontalHeaderItem(1)
        ___qtablewidgetitem1.setText(QCoreApplication.translate("MainWindow", u"2", None));
        ___qtablewidgetitem2 = self.tableWidget.verticalHeaderItem(0)
        ___qtablewidgetitem2.setText(QCoreApplication.translate("MainWindow", u"1", None));
        ___qtablewidgetitem3 = self.tableWidget.verticalHeaderItem(1)
        ___qtablewidgetitem3.setText(QCoreApplication.translate("MainWindow", u"2", None));
        ___qtablewidgetitem4 = self.tableWidget.verticalHeaderItem(2)
        ___qtablewidgetitem4.setText(QCoreApplication.translate("MainWindow", u"3", None));
        self.pushButton.setText(QCoreApplication.translate("MainWindow", u"\u041f\u043e\u0441\u0442\u0440\u043e\u0438\u0442\u044c \u0433\u0440\u0430\u0444\u0438\u043a", None))
        self.tabWidget.setTabText(self.tabWidget.indexOf(self.tab), QCoreApplication.translate("MainWindow", u"\u0422\u0430\u0431\u043b\u0438\u0446\u0430", None))
        self.tabWidget.setTabText(self.tabWidget.indexOf(self.tab_2), QCoreApplication.translate("MainWindow", u"\u0413\u0440\u0430\u0444\u0438\u043a", None))


class TimeAxisItem(pg.AxisItem):                                  # +++
    def tickStrings(self, values, scale, spacing):
        return [datetime.fromtimestamp(value) for value in values]
   

class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):   
    def __init__(self):
        super().__init__()
        
        self.setupUi(self)
        self.pushButton.clicked.connect(self.button_clicked)
        
        for row in range(self.tableWidget.rowCount()):
            hours, minutes = 15 + row, 12 + (row + 15)
            date_from = QtWidgets.QDateTimeEdit()
            date_from.setDateTime(
                QtCore.QDateTime(
                    QtCore.QDate(2021, 9, 7), 
                    QtCore.QTime(hours, minutes, 0)
                )
            )
            self.tableWidget.setCellWidget(row, 0, date_from) 

            item = QTableWidgetItem()
            _data = 5 * row
            item.setData(Qt.DisplayRole, _data)    
            self.tableWidget.setItem(row, 1, item)              

    def button_clicked(self):
        data_for_word = []
        rows = self.tableWidget.rowCount()
        for row in range(rows):
            dateTime = self.tableWidget.cellWidget(row, 0).\
                       dateTime().toString('dd.MM.yyyy hh:mm')                      
            item = self.tableWidget.item(row, 1)
            _data = item.data(Qt.DisplayRole) if item else '0'
            data_for_word.append([dateTime, _data])
        
        self.x, self.y = [], []
        for x, y in data_for_word:
            self.x.append(datetime.strptime(x, '%d.%m.%Y %H:%M'))
            self.y.append(float(y))
# !!! ВНИМАНИЕ
        self.date_axis = TimeAxisItem(orientation='bottom')
        self.widget = pg.PlotWidget(
            axisItems = {'bottom': self.date_axis},
            title= '<h2>Hello World</h2>'
        )
        pen = pg.mkPen(color=(255, 0, 0), width=2)
        self.widget.plot(
            x=[x.timestamp() for x in self.x], 
            y=self.y, pen=pen, symbol='o'
        )
        self.gridLayout_2.addWidget(self.widget, 0, 0, 1, 1)
# !!!

if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.resize(640, 480)
    w.show()
    sys.exit(app.exec_())