How can I change my plot so that each graph is a different color?

Question

I'm confused about how I can change the colors of graphs in the plots, each line represents an approximation of Euler with different h value.

import numpy as np
import matplotlib.pylab as plt

# your function
def Eulergraph(h, N, ax):
    K = 12; r = 0.43; Po = 1;

#defining dP/dt as a function f(P)
    f = lambda P: r*P*(1-P/K)

    P = np.array([])
    P = np.append(P,Po) #initializing P with Po

    for n in range(N+1):
        Pn = P[n] + h*f(P[n])
        P = np.append(P,Pn)





# formatting of your plot
plt.xlabel (' Value of n ”' )
plt.ylabel (" Value of p[n] ”")
plt.title (" Approximate Solution with Euler’s Method " )
plt.show()

Right leg · Accepted Answer · 2019-05-02 22:11:40Z

2

You just have to take the ax.plot call outside of the for loop, and just plot P without n, without forcing a red color with the 'r' flag, just like that:

for n in range(N+1):
    Pn = P[n] + h*f(P[n])
    P = np.append(P,Pn)

ax.plot(P, 'o')

In your original code, you plot each point independently. This is not necessary, as matplotlib can directly plot a vector, or a list for that matter. So you can simply populate P, and then plot it without X data.

The 'ro' options mean:

Draw red markers (r)
Use round markers (o)

If you remove the color option and simply pass o, matplotlib will take care of drawing each function in a distinct color.

edited May 2, 2019 at 22:11

answered May 2, 2019 at 22:06

Right leg

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Sheldore · Accepted Answer · 2019-05-02 22:42:16Z

While @Right leg already pointed out the problem, you might be interested in knowing how to get legends.

import numpy as np
import matplotlib.pylab as plt

# your function
def Eulergraph(h, N, ax):
    K = 12; r = 0.43; Po = 1;
    f = lambda P: r*P*(1-P/K)
    P = np.array([Po]) # Modified this line

    for n in range(N+1):
        Pn = P[n] + h*f(P[n])
        P = np.append(P,Pn)
    ax.plot (P, '-', label='h=%s' %h) # Added legend here

# create your figure and axis object
fig = plt.figure()
ax = plt.gca()

# pass the axis object as a parameter
Eulergraph(1,30,ax)       
Eulergraph(.5,30,ax)   
Eulergraph(.1,30,ax)

# formatting of your plot
plt.xlabel (' Value of n ”' )
plt.ylabel (" Value of p[n] ”")
plt.title (" Approximate Solution with Euler’s Method " )
plt.legend() # Show the legend 
plt.show()

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