Add cfg flag to plot
//! let mut y = x.fmap(|t| t.powi(2));
//! y = zip_with(|a, b| a + b, &y, &normal2.sample(y.len()));
//!
-//! // Initial paramter
+//! // Initial parameter
//! let n_init = vec![1f64];
//! let data = hstack!(x.clone(), y.clone());
//!
//! let z = quad(&x, NumberVector::from_f64_vec(p)).to_f64_vec();
//!
//! // Plot
+//! #[cfg(feature = "plot")]
+//! {
//! let mut plt = Plot2D::new();
//! plt.set_domain(x)
//! .insert_image(y) // plot data
//! .set_marker(vec![Point, Line])
//! .savefig().expect("Can't draw a plot");
//! }
+//! }
//!
//! fn quad(x: &Vec<f64>, n: Vec<Number>) -> Vec<Number> {
//! x.clone().into_iter()
