matplotlib, no plot when using Figure

[ajvperth]

Hello,
as I am breaking by brains about this, I hope you can give me a suggestion.
I work on Linux / openSUSE 15.3 and python 3.9.4 in a virtual console.
I can not understand why this is not providing a plot output

import matplotlib.pyplot as plt
from matplotlib.figure   import Figure

fig = Figure(figsize=(5, 4), dpi=100)           # Create a in memory figure
ax  = fig.add_subplot(111)                      # Create a axes in fig
ax.plot([1, 2, 3, 4], [1, 4, 2, 3])
plt.show()                                      # Show in X

and this does

fig, ax = plt.subplots()               
ax.plot([1, 2, 3, 4], [1, 4, 2, 3])
plt.show() 

Thanks, Anthony

[deanhystad]

import matplotlib.pyplot as plt

fig = plt.figure()
ax  = fig.add_subplot(1, 1, 1)  # or 111
ax.plot([1, 2, 3, 4], [1, 4, 2, 3])
plt.show()

Why does this work for plt.figure() but not matplotlib.figure.Figure()? So I wrote this program and inspect a and b.

import matplotlib.pyplot as plt
from matplotlib.figure import Figure

a = plt.figure()
b = Figure()
plt.show()  # Just needed a line to set a breakpoint

a and b look similar except for "canvas"
a.canvas =canvas:<matplotlib.backends.backend_tkagg.FigureCanvasTkAgg object at 0x000001D8E2594D30>
b.canvas = canvas:<matplotlib.backend_bases.FigureCanvasBase object at 0x000001D8E5DB1D00>

So it appears that matplotlib.pyplot.figure creates a matplotlib.figure.Figure() and sets the canvas to a FigureCanvasTkAgg type.

For more information about agg backend you can look here:

https://matplotlib.org/stable/gallery/us...asagg.html

[ajvperth]

Dear DeanHystad,

you are absolutely right with your answer.
I checked it in the the source code.
b = Figure() : is a figure in memory only
a = plt.figure() : is a same figure but linked to a canvas (screen frame buffer)