def load(self, requirements_file=None):
        """Load or reload requirements from a requirements.txt file.

        :param requirements_file: if not given, the filename used from
            initialization will be read again.
        """
        if requirements_file is None:
            requirements_file = self.filename
            if requirements_file is None:
                raise ValueError("no filename provided")
        elif isinstance(requirements_file, text_type):
            requirements_file = Path(requirements_file)
        self.clear()
        with requirements_file.open() as f:
            self.loads(f.read())

        if isinstance(requirements_file, (text_type, Path)):
            self.filename = requirements_file
def forcedens_surface(fdens, ax=0):
    if ax == 0:
        _create_3d_axis()
        ax = plt.gca()
    xpos = [p for p in fdens.positions[0]['X']]
    ypos = [p['position'] for p in fdens.positions]
    z = 1e-3*np.array([p['FN']
                       for p in fdens.positions])
    _plot_surface(ax, xpos, ypos, z,
                  (u'Rotor pos/°', u'Pos/°', u'F N / kN/m²'))