from matplotlib.patches import Rectangle

import matplotlib.pyplot as plt
import numpy as np

def create_plot(f, xs, x0, eps, delta, filename):
    ys = f(xs)
    y0 = f(x0)

    xmin = xs.min()
    ymin = ys.min()

    xss = xs[(xs > x0-delta) & (xs < x0+delta)]
    yss = f(xss)

    sg = (yss > y0-eps) & (yss < y0+eps)

    fig, ax = plt.subplots(1,1)

    ax.add_patch(
        Rectangle((x0-delta,y0-eps), 2*delta, 2*eps, edgecolor="black",
                  alpha=0.6, linestyle="solid", facecolor="#dddddd", lw=1.5))
    ax.plot(xs, ys)

    yss[~sg] = None
    ax.plot(xss, yss, color="green", lw=2)

    yss = f(xss)
    yss[sg] = None
    ax.plot(xss, yss, color="red", lw=2)

    ax.plot([x0], [y0], "o", color="black")

    fig.savefig(filename)

xs = np.linspace(-2, 4, 1e5)

create_plot(lambda x: x/3.0, xs, 1, 1, 2, "Visualization of epsilon-delta-continuity 1.svg")
create_plot(lambda x: x/3.0, xs, 1, 0.5, 1, "Visualization of epsilon-delta-continuity 2.svg")
create_plot(lambda x: x/3.0, xs, 1, 1./3, 3./4, "Visualization of epsilon-delta-continuity 3.svg")
create_plot(lambda x: x/3.0, xs, 1, 0.5, 2, "Visualization of epsilon-delta-continuity 4.svg")