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For an upcoming assignment I am require to make a series of diagrams that have two graphs that have a line going across from one graph to the other, colouring an area below that line on the other graph.

As shown in this rough drawing:

enter image description here

This is what I currently have: enter image description here

From this code:

from matplotlib import pyplot as plt

fig = plt.figure()
ax1 = fig.add_subplot(121)
ax1.plot([0,1,2,3,4,5,6,7,8,9,10], [1,1,1,1,1,0,0,0,0,0,0], '-b')
ax1.plot([0,1,2,3,4,5,6,7,8,9,10], [0,0,0,0,0,1,1,1,1,1,1], '-r')
ax1.set_ylim([0, 1.2])

ax2 = fig.add_subplot(122)
ax2.plot([0,5,10,15,20,25,30,35,40], [1,1,1,1,0,0,0,0,0], '-b')
ax2.plot([0,5,10,15,20,25,30,35,40], [0,0,0,0,1,1,1,1,1], '-r')
ax2.set_ylim([0, 1.2])

plt.show()

Obviously this only generates the two graphs and I have yet been unable to add the line across the two graphs.

I really want to be able to do this with Matplotlib in python with the ability to change the value (45 in the example case) and the coloured area change automatically.

Thanks!

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  • What code have you tried so far? Commented Apr 3, 2014 at 14:29
  • Welcome to Stack Overflow! It looks like you want us to write some code for you. While many users are willing to produce code for a coder in distress, they usually only help when the poster has already tried to solve the problem on their own. A good way to demonstrate this effort is to include the code you've written so far, example input (if there is any), the expected output, and the output you actually get (console output, stack traces, compiler errors - whatever is applicable). The more detail you provide, the more answers you are likely to receive. Check the FAQ and How to Ask Commented Apr 3, 2014 at 14:34

1 Answer 1

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There are three steps: 1st, find the intercept point between green and blue lines in the left panel. 2nd, find the intercept point between the red and lines in the right panel. 3rd, fill the area between. These steps involves np.interp scipy.interpolat scipy.optimize and plt.fill_between, which you should look up.

from matplotlib import pyplot as plt
import numpy as np
import scipy.interpolate as spinp
import scipy.optimize as spop

fig = plt.figure(figsize=(16,4))
ax1 = fig.add_subplot(121)
x  = [0,10,20,30,40,50,60,70,80,90,100]
yr = [1,1,1,1,1,0,0,0,0,0,0]
yg = [0,0,0,0,0,1,1,1,1,1,1]
turn_pt = np.interp(45, x, yr) #change 45 to whatever.
yb = [0.,turn_pt,turn_pt]
ax1.plot(x, yr, '-r')
ax1.plot(x, yg, '-g')
xb = [45, 45, 200]
ll = plt.plot(xb,yb, '-b')
ll[0].set_clip_on(False) 
plt.axis([0,100,0,1.2])
#the above three lines to draw the line out of the box.

ax2 = fig.add_subplot(122)
yr = [1,1,1,1,0,0,0,0,0]
yg = [0,0,0,0,1,1,1,1,1]
x  = [0,5,10,15,20,25,30,35,40]
brk_pt_f = lambda X, V: (spinp.interp1d(x, yr)(X)-V)**2
brk_pt = spop.fmin(brk_pt_f, 17., args=(turn_pt,), disp=0) #17. is you inital guess,
#the above two lines solve for the intersection between the blue line and the red line
zero_pt = 20.
start_pt= 0.
xb = np.hstack((start_pt, brk_pt, zero_pt))
yb = [turn_pt,turn_pt,0]
ax2.plot(x, yr, '-r')
ax2.plot(x, yg, '-g')
ax2.plot(xb, yb, '-b')
ax2.hlines(turn_pt,0, 40, 'b', alpha=0.)
ax2.fill_between(xb, yb, 0, alpha=0.4)
ax2.set_ylim([0, 1.2])
ax2.set_xlim([0, 40])

enter image description here

There are a few solutions to get rid of the top x-axis and the right y-axis, please search older SO posts.

And finally, welcome to SO.

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