Hello all,
I’m looking at embedding a matplotlib figure in a Tkinter program, but looking at the example given in the matplotlib user guide I can’t work out what it’s actually meant to graph (I appreciate some would consider this more of a raw maths question than a Python one). Specifically I can’t work out what the call to ax.plot() on the 16th line is supposed to be plotting.
I can see that t is a numpy.ndarray object containing values ranging from 0 to 2.9 inclusive in increments of 0.1, so it contains 30 discrete values. I’ve worked out that multiplying a np.array object by x will multiply each member of the array by x (as opposed to duplicating the array x times as would be the case with a standard python list). Given the presence of np.sine() and 2* pi I’ve a feeling it relates to radians and trigonometry but I can’t work out what it’s supposed to graph.
Of course the x axis is labeled time [s] and the y axis is f(t) which, considering the handler update_frequency() might be frequency(time).
At first I thought it was meant to graph the sine value of angles encompassing 1/10th of a circle, 2/10th’s of a circle, etc. But the values in t exceed 1 so that doesn’t make sense - plus the value returned by np.sine() is multiplied by 2, and none of that relates to frequency or time in any obvious way. I wish the author had just included a comment explaining what the code is supposed to plot 
If someone could point me in the right direction I’d be super grateful! I’ve posted the example from the matplotlib user guide below.
import tkinter
import numpy as np
# Implement the default Matplotlib key bindings.
from matplotlib.backend_bases import key_press_handler
from matplotlib.backends.backend_tkagg import (FigureCanvasTkAgg,
NavigationToolbar2Tk)
from matplotlib.figure import Figure
root = tkinter.Tk()
root.wm_title("Embedding in Tk")
fig = Figure(figsize=(5, 4), dpi=100)
t = np.arange(0, 3, .01)
ax = fig.add_subplot()
line, = ax.plot(t, 2 * np.sin(2 * np.pi * t))
ax.set_xlabel("time [s]")
ax.set_ylabel("f(t)")
canvas = FigureCanvasTkAgg(fig, master=root) # A tk.DrawingArea.
canvas.draw()
# pack_toolbar=False will make it easier to use a layout manager later on.
toolbar = NavigationToolbar2Tk(canvas, root, pack_toolbar=False)
toolbar.update()
canvas.mpl_connect(
"key_press_event", lambda event: print(f"you pressed {event.key}"))
canvas.mpl_connect("key_press_event", key_press_handler)
button_quit = tkinter.Button(master=root, text="Quit", command=root.destroy)
def update_frequency(new_val):
# retrieve frequency
f = float(new_val)
# update data
y = 2 * np.sin(2 * np.pi * f * t)
line.set_data(t, y)
# required to update canvas and attached toolbar!
canvas.draw()
slider_update = tkinter.Scale(root, from_=1, to=5, orient=tkinter.HORIZONTAL,
command=update_frequency, label="Frequency [Hz]")
# Packing order is important. Widgets are processed sequentially and if there
# is no space left, because the window is too small, they are not displayed.
# The canvas is rather flexible in its size, so we pack it last which makes
# sure the UI controls are displayed as long as possible.
button_quit.pack(side=tkinter.BOTTOM)
slider_update.pack(side=tkinter.BOTTOM)
toolbar.pack(side=tkinter.BOTTOM, fill=tkinter.X)
canvas.get_tk_widget().pack(side=tkinter.TOP, fill=tkinter.BOTH, expand=True)
tkinter.mainloop()
