The same variable should be used in all univariate expressions being plotted.

Raise code

 = list(map(sympify, args))
    free = set()
    for a in args:
        if isinstance(a, Expr):
            free |= a.free_symbols
            if len(free) > 1:
                raise ValueError(
                    'The same variable should be used in all '
                    'univariate expressions being plotted.')
    x = free.pop() if free else Symbol('x')
    kwargs.setdefault('xlabel', x.name)
    kwargs.setdefault('ylabel', 'f(%s)' % x.name)
    series = []
    plot_expr = check_arguments(args, 1, 1)
    series = [Li