message: Fix doc tests for without gnuplot.
diff: //! use plot::prelude::*;
  //!
  //! # fn main() {
  +//! # if let Err(_) = plot::version() {
  +//! # return;
  +//! # }
  //! let ref xs = linspace::<f64>(-10., 10., 51).collect::<Vec<_>>();
  //!
  //! # fs::create_dir_all(Path::new("target/doc/criterion_plot")).unwrap();
  //! }
  //!
  //! # fn main() {
  +//! # if let Err(_) = plot::version() {
  +//! # return;
  +//! # }
  //! let ref xs_ = linspace::<f64>(-4., 4., 101).collect::<Vec<_>>();
  //!
  //! // Fake some data
  //! use rand::Rng;
  //!
  //! # fn main() {
  +//! # if let Err(_) = plot::version() {
  +//! # return;
  +//! # }
  //! let xs = 1..11;
  //!
  //! // Fake some data
  //! }
  //!
  //! # fn main() {
  +//! # if let Err(_) = plot::version() {
  +//! # return;
  +//! # }
  //! let (start, end): (f64, f64) = (1.1, 90_000.);
  //! let ref xs = linspace(start.ln(), end.ln(), 101).map(|x| x.exp()).collect::<Vec<_>>();
  //! let phase = xs.iter().map(|&x| tf(x).arg() * 180. / PI);
  //! use plot::prelude::*;
  //!
  //! # fn main() {
  +//! # if let Err(_) = plot::version() {
  +//! # return;
  +//! # }
  //! let (start, end) = (-5., 5.);
  //! let ref xs = linspace(start, end, 101).collect::<Vec<_>>();
  //! let zeros = iter::repeat(0);
  