code: def plot_trajectory_projection(self, x_axis, y_axis,
                                     legend=False, ax=None,
                                     start_and_end_locations_only=False,
                                     start_marker='bo',
                                     end_marker='rx',
                                     *args, **kwargs):
          
  
          x, y = self.solutions_as_2d_trajectories(x_axis, y_axis)
  
          plot_2d_trajectory(x, y,
                             x_label=x_axis, y_label=y_axis,
                             legend=legend,
                             ax=ax,
                             start_and_end_locations_only=start_and_end_locations_only,
                             start_marker=start_marker,
                             end_marker=end_marker,
                             *args, **kwargs)
docs: Plots the projection of the trajectory through the parameter space the minimisation algorithm took.
  
          Since parameter space is often high-dimensional and paper can realistically represent only two,
          one needs to specify the ``x_axis``, and ``y_axis`` arguments with the variable names to project the
          high-dimensional grid, on, i.e. ``x_axis='c_1'``, ``y_axis='c_4'``
  
          :param x_axis: variable name (parameter or left hand side of equation) to project x axis onto
          :type x_axis: str|:class:`~sympy.Symbol`
          :param y_axis: variable name to project y axis onto
          :type y_axis: str|:class:`~sympy.Symbol`
          :param legend: Whether to display legend or not
          :type legend: bool
          :param ax: Axis to plot onto (defaults to :func:`matplotlib.pyplot.gca` if not set)
          :param start_and_end_locations_only: If set to true, will not plot the trajectory, but only start and end parameter
          :param start_marker: The marker to use for start of trajectory, defaults to blue circle
          :param end_marker: The marker to use for end of trajectory, defaults to red x
          :param args: Arguments to pass to :func:`matplotlib.pyplot.plot` function
          :param kwargs: Keyword arguments to pass to :func:`matplotlib.pyplot.plot` function