Beginner's questions about Matplotlib OO Api - matplotlib

Question

I am new to Matplotlib and am struggling a bit to differentiate between the OO and pyplot interfaces. I’m actually working with the Kivy GUI framework and trying to plot 4 subplots on a single figure, to be displayed by Kivy. Here’s a snippet of my code:
def create_plot(self):
self.fig, ((self.ax0, self.ax1), (self.ax2, self.ax3)) = plt.subplots(nrows=2, ncols=2)
self.ax0.set_title("A")
self.ax0.grid(True, lw = 2, ls = '--', c = '.75')
self.ax1.set_title("B")
self.ax1.grid(True, lw = 2, ls = '--', c = '.75')
self.ax2.set_title("C")
self.ax2.grid(True, lw = 2, ls = '--', c = '.75')
self.ax3.set_title("D")
self.ax3.grid(True, lw = 2, ls = '--', c = '.75')
#plt.tight_layout()
plt.show()
canvas = self.fig.canvas
self.add_widget(canvas)
What worries me is that I am calling plt methods and assigning the results to my objects. Is plt the state machine interface and not the OO interface, or is this OK?
Secondly, I want to periodically update the plotted lines, so I have a plot method that does this:
def plot(self, xCoords, yCoords):
if len(self.ax0.lines) > 0:
self.ax0.lines.pop(0)
line = self.ax0.plot(xCoords, yCoords, color='blue')
canvas = self.fig.canvas
canvas.draw()
Does that look ok? Can I just pop the existing line, or should I reuse the existing line?
Lastly, and most difficult, if I enable:
plt.tight_layout()
I get an exception:
C:\Kivy-1.9.0-py3.4-win32-x64\Python34\lib\site-packages\matplotlib\tight_layout.py:225: UserWarning: tight_layout : falling back to Agg renderer
warnings.warn("tight_layout : falling back to Agg renderer")
Traceback (most recent call last):
File "main.py", line 1117, in <module>
GuiApp().run()
File "C:\Kivy-1.9.0-py3.4-win32-x64\Python34\lib\site-packages\kivy\app.py", line 801, in run
self.load_kv(filename=self.kv_file)
File "C:\Kivy-1.9.0-py3.4-win32-x64\Python34\lib\site-packages\kivy\app.py", line 598, in load_kv
root = Builder.load_file(rfilename)
File "C:\Kivy-1.9.0-py3.4-win32-x64\Python34\lib\site-packages\kivy\lang.py", line 1801, in load_file
return self.load_string(data, **kwargs)
File "C:\Kivy-1.9.0-py3.4-win32-x64\Python34\lib\site-packages\kivy\lang.py", line 1880, in load_string
self._apply_rule(widget, parser.root, parser.root)
File "C:\Kivy-1.9.0-py3.4-win32-x64\Python34\lib\site-packages\kivy\lang.py", line 2038, in _apply_rule
self._apply_rule(child, crule, rootrule)
File "C:\Kivy-1.9.0-py3.4-win32-x64\Python34\lib\site-packages\kivy\lang.py", line 2037, in _apply_rule
self.apply(child)
File "C:\Kivy-1.9.0-py3.4-win32-x64\Python34\lib\site-packages\kivy\lang.py", line 1924, in apply
self._apply_rule(widget, rule, rule)
File "C:\Kivy-1.9.0-py3.4-win32-x64\Python34\lib\site-packages\kivy\lang.py", line 2038, in _apply_rule
self._apply_rule(child, crule, rootrule)
File "C:\Kivy-1.9.0-py3.4-win32-x64\Python34\lib\site-packages\kivy\lang.py", line 2038, in _apply_rule
self._apply_rule(child, crule, rootrule)
File "C:\Kivy-1.9.0-py3.4-win32-x64\Python34\lib\site-packages\kivy\lang.py", line 2035, in _apply_rule
child = cls(__no_builder=True)
File "C:\SVNProj\Raggio\trunk\hostconsole\gui\mygraph.py", line 127, in __init__
self.create_plot()
File "C:\SVNProj\Raggio\trunk\hostconsole\gui\mygraph.py", line 224, in create_plot
self.add_widget(canvas)
File "C:\Kivy-1.9.0-py3.4-win32-x64\Python34\lib\site-packages\kivy\uix\boxlayout.py", line 211, in add_widget
widget.bind(
AttributeError: 'FigureCanvasAgg' object has no attribute 'bind'
Can anyone help with that please?
Best regards
David

Answer 1

Original post
regarding def create_plot
plt.subplots() is just a wrapper around creating a new figure and adding subplots (axes) to that figure, so that is safe to use in any context AFAIK
regarding updating lines vs creating new lines.
Popping off and creating a new line with a call to axes.plot() will work, but will be slower than just using the existing line artist and calling artist.set_data(x_data, y_data)
Regarding the tight_layout issue
Consider just calling self.fig.tight_layout() instead of plt.tight_layout(), as plt.tight_layout() might be grabbing the wrong figure
Update based on discussion in comments:
Since you are plotting one line per panel, I would suggest adding these four lines to your create_plot method to save the LineArtist's that are created by the axes.plot() method:
self.line0, = self.ax0.plot([], [], 'b')
self.line1, = self.ax1.plot([], [], 'b')
self.line2, = self.ax2.plot([], [], 'b')
self.line3, = self.ax3.plot([], [], 'b')
Then in the plot method you can just do this:
self.line0.set_data(xCoords, yCoords)
Though I seem to be struggling to remember how to make ax0 automatically update its limits based on the xCoords and yCoords. I thought that it was as simple as self.ax0.relim(), but that is not working in my jupyter notebook right now. Hmm.