matplotlib + PyQt5: plot area margins

Question

I am trying to build simple user interface with a graph. The combination of matplotlib and PyQt5 works good, but there is one thing I don't like: bulky margins around axes area.

On the image that I have attached: top and bottom margins taking almost third part of all area. How can I make them smaller?

image

Code I using looks like:

from PyQt5 import QtCore, QtWidgets, QtGui
import matplotlib
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure
from matplotlib import dates

class GraphWin(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        QtWidgets.QMainWindow.__init__(self, parent)

        main_layout = QtWidgets.QHBoxLayout(self)

        # graph area
        self.graph = Figure()
        self.graph.autofmt_xdate()
        self.axis = self.graph.add_subplot(111)
        self.canvas = FigureCanvas(self.graph)
        main_layout.addWidget(self.canvas)

        self.draw()

    # draw function
    def draw(self):
        self.axis.plot(some_x, some_y, some_color, label=some_label)
        fmt = dates.DateFormatter('%M:%S')
        self.axis.xaxis.set_major_formatter(fmt)
        self.axis.xaxis.set_major_locator(matplotlib.ticker.MaxNLocator(7))
        self.axis.legend(loc=2)
        self.axis.grid(b=True, which='both', color='#cccccc', linestyle='--')
        self.canvas.draw()

This question differs from this because I would like to know how to adjust plot margins absolutely, e.g. in pixels or inches/cm not in percents like using function subplots_adjust().