Serial port data plotter in PyQt
Instead of inheriting from ui_main.Ui_GraphPlotter, I would use the PyQt4.uic module to dynamically load your ui file in the __init__ method:

import os
import pyqtgraph

from PyQt4 import QtGui
from PyQt4 import uic

curdir = os.path.dirname(os.path.abspath(__file__))


class GraphPlotter(QtGui.QMainWindow):

    def __init__(self):
        super().__init__()
        uic.loadUi(os.path.join(curdir, 'GraphPlotter.ui'), self)
        pyqtgraph.setConfigOption('background', 'w')
        # ...