Calculate area in polygon

polyarea.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Tue Mar  5 20:17:08 2019

@author: bill
"""

import matplotlib.pyplot as plt
import numpy as np

import matplotlib
matplotlib.use('Qt5Agg')

fig, ax = plt.subplots()
plt.imshow(plt.imread('curve.png'))

points = []

def onclick(event):
    print('button=%d, x=%d, y=%d, xdata=%f, ydata=%f' %
          (event.button, event.x, event.y, event.xdata, event.ydata))
    plt.plot(event.xdata, event.ydata, 'x', markersize=50)
    points.append((event.xdata, event.ydata))
    fig.canvas.draw()

cid = fig.canvas.mpl_connect('button_press_event', onclick)

def polyArea(x,y):
    """
    https://stackoverflow.com/a/30408825/6610243
    """
    return 0.5*np.abs(np.dot(x,np.roll(y,1))-np.dot(y,np.roll(x,1)))

print(polyArea(*zip(*points)))

Picking points on Matplotlib: https://stackoverflow.com/a/41825214/6610243

-1 for non-pythonic use of global scoped variables.