def __call__(self, t=None, x=None, r=None, y=None, z=None, R=None, warn=True):
        """
        Evaluate the variable at arbitrary *dimensional* t (and x, r, y, z and/or R),
        using interpolation
        """
        kwargs = {"t": t, "x": x, "r": r, "y": y, "z": z, "R": R}
        # Remove any None arguments
        kwargs = {key: value for key, value in kwargs.items() if value is not None}
        # Use xarray interpolation, return numpy array
        return self._xr_data_array.interp(**kwargs).values

# variable that depends on a  spatial variable named "s" and time "t"
my_var_data_array = sim.solution["My var"]._xr_data_array

def my_var_eval(s, t):
    return my_var_data_array.interp(s=s, t=t).values