update plot after creating it

Question

In the textbook about Python I am working with I read: "Every visual aspect of a graph can be changed from its default. You can specify these when you create a plot; most can also be changed later."

So creating a .py file

# simple_plot.py
import numpy as np, matplotlib.pyplot as plt
num_points = 5
x_min, x_max = 0, 4
5 x_values = np.linspace(x_min, x_max, num_points)
y_values = x_values**2
plt.plot(x_values, y_values)

and running it, gives the desired plot. Now typing in the console e.g.

plt.plot(x_values,y_values,'r--o')

plots a red dashed line with red circles at each point. However I cannot understand why typing in the console (and not adding the instructions in the script created initially) something like

plt.title("My first plot", size=24, weight='bold')
plt.xlabel("speed")
plt.ylabel("kinetic energy")

does not update the plot. Thank you very much.

Have you tried using plt.draw() after those last three lines? — Spherical Cowboy
– Spherical Cowboy, Commented Feb 25, 2017 at 14:44
What exactly do you mean by "the console"? Is it the python console you are talking about or IPython console or IPython notebook? How do you run the script? — ImportanceOfBeingErnest
– ImportanceOfBeingErnest, Commented Feb 25, 2017 at 15:04
@ spherical cowboy : Still with plt.draw() I don't see something different. — Dimitris
– Dimitris, Commented Feb 25, 2017 at 21:12
@ImportanceOfBeingErnest : I am talking about IPython console. Sorry for not being precise. — Dimitris
– Dimitris, Commented Feb 25, 2017 at 21:15

ImportanceOfBeingErnest · Accepted Answer · 2017-02-25 22:06:02Z

I'm assuming that you run your script (let's call it myscript.py) by calling run myscript.py in the IPython console, with the %matplotlib inline option activated.

This will plot the figure as desired into the console. However once the figure is drawn you loose the reference to it. Calling plt.plot(x_values,y_values,'r--o') creates a new figure, and plt.title(..) yet another one.

One thing you could do is work more in the object oriented way. I.e. create the figure and keep a reference in the myscript.py file like this:

import numpy as np, matplotlib.pyplot as plt
num_points = 5
x_min, x_max = 0, 4
x_values = np.linspace(x_min, x_max, num_points)
y_values = x_values**2

fig, ax = plt.subplots()
ax.plot(x_values, y_values)

then in the console you may type

ax.set_title("My Title")
ax.set_xlabel("MyLabel")

and show the figure with the updated properties anytime you like typing fig.

It is exactly what I was looking for. Not just a workaround but a clear and concise explanation. Thank you very much for your time!

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