#!/usr/bin/env python3
#
# linear_function.py
#


from matplotlib.ticker import FuncFormatter
from matplotlib.ticker import MultipleLocator
import matplotlib.pyplot as plt
import numpy as np


def main():

    fig, ax = plt.subplots(figsize=(8,8))

    ax = configure_axes(ax, 'Linear Function', 10, 10)

    x = np.linspace(-4, 4, 100)
    y = 2 * x + 3/2
    ax.plot(x, y, color='b')

    x = np.linspace(-9, 9, 100)
    y = - 1/2 * x - 3
    ax.plot(x, y, color='r')

    ax.text(5, 9,  r'$y = 2x + \frac{3}{2}}$',  color='b', horizontalalignment='left',  verticalalignment='bottom')
    ax.text(9, -8, r'$y = -\frac{1}{2}x - 3}$', color='r', horizontalalignment='right', verticalalignment='top')

    fig.tight_layout()

    plt.show()
    # end of main()


def configure_axes(ax, title, xlimit, ylimit, xmajorunit = 5, xminorunit = 1, ymajorunit = 5, yminorunit = 1):

    ax.set_title(title, fontsize='xx-large', loc='left')

    ax.spines['left'].set_position('zero')
    ax.spines['right'].set_visible(False)
    ax.spines['bottom'].set_position('zero')
    ax.spines['top'].set_visible(False)

    ax.text(-0.2, -0.2, 'O', fontsize='x-large', fontstyle='italic', horizontalalignment='right', verticalalignment='top')

    xlimit += xmajorunit*0.8
    ylimit += ymajorunit*0.8
    ax.set_xlim((-xlimit, xlimit))
    ax.set_ylim((-ylimit, ylimit))

    ax.plot(1, 0, '>k', transform=ax.get_yaxis_transform(), clip_on=False)
    ax.xaxis.set_label_text('x')
    ax.xaxis.set_label_coords(1.01, 0.48)
    ax.xaxis.set_major_formatter(FuncFormatter(lambda w, pos: int(w) if w else ''))
    ax.xaxis.set_major_locator(MultipleLocator(xmajorunit))
    ax.xaxis.set_minor_locator(MultipleLocator(xminorunit))
    ax.xaxis.set_tick_params(which='both', direction='in')

    ax.plot(0, 1, '^k', transform=ax.get_xaxis_transform(), clip_on=False)
    ax.yaxis.set_label_text('y')
    ax.yaxis.set_label_coords(0.48, 1)
    ax.yaxis.label.set_rotation(0)
    ax.yaxis.set_major_formatter(FuncFormatter(lambda w, pos: int(w) if w else ''))
    ax.yaxis.set_major_locator(MultipleLocator(ymajorunit))
    ax.yaxis.set_minor_locator(MultipleLocator(yminorunit))
    ax.yaxis.set_tick_params(which='both', direction='in')

    return ax
    # end of configure_axes()


if __name__ == '__main__': main()