Q:

multithreading matplotlib graph in a kivy app

I wish to generate several graphs by using matplotlib when i'm leaving my kivy app. 
Problem
Generate 4-6 graphs in .png takes time (around 1 or 2 minutes in my case), that's why I wished to multithread my process but it didn't work (got the exceptionError below...)
[WARNING] [generer graph] chemin : /media/pi/DATA8/2019-3-15-10-40-26/data_dut_1_2019-3-15-10-40-26.csv
[WARNING] [generer_graph] nom_graph : /media/pi/DATA8/2019-3-15-10-40-26/data_dut_1_2019-3-15-10-40-26.png
[WARNING] [generer graph] chemin : /media/pi/DATA8/2019-3-15-10-40-26/data_dut_2_2019-3-15-10-40-26.csv
[WARNING] [generer graph] chemin : /media/pi/DATA8/2019-3-15-10-40-26/data_dut_3_2019-3-15-10-40-26.csv
[WARNING] [generer_graph] nom_graph : /media/pi/DATA8/2019-3-15-10-40-26/data_dut_2_2019-3-15-10-40-26.png
[WARNING] [generer_graph] nom_graph : /media/pi/DATA8/2019-3-15-10-40-26/data_dut_3_2019-3-15-10-40-26.png
[WARNING] [generer graph] chemin : /media/pi/DATA8/2019-3-15-10-40-26/data_dut_4_2019-3-15-10-40-26.csv
[WARNING] [generer_graph] nom_graph : /media/pi/DATA8/2019-3-15-10-40-26/data_dut_4_2019-3-15-10-40-26.png
 Exception in thread Thread-4:
 Traceback (most recent call last):
 Exception in thread Thread-3:
   File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
 Traceback (most recent call last):
     self.run()
 Exception in thread Thread-6:
   File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
   File "interfacebancdetest_dev/main.py", line 217, in run
     self.run()
 Traceback (most recent call last):
     genGraph.generer_graph(chemin=self.chemin, y_low_min=self.y_low_min, y_low_max=self.y_low_max, y_high_min=self.y_high_min, y_high_max=self.y_high_max, format_graph=self.format_graph)
   File "interfacebancdetest_dev/main.py", line 217, in run
   File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
   File "/home/pi/Desktop/interfacebancdetest_dev/genGraph.py", line 48, in generer_graph
     genGraph.generer_graph(chemin=self.chemin, y_low_min=self.y_low_min, y_low_max=self.y_low_max, y_high_min=self.y_high_min, y_high_max=self.y_high_max, format_graph=self.format_graph)
     self.run()
     plt.plot(abcisse, ordonnee, marker=',')
   File "/home/pi/Desktop/interfacebancdetest_dev/genGraph.py", line 48, in generer_graph
   File "interfacebancdetest_dev/main.py", line 217, in run
     plt.plot(abcisse, ordonnee, marker=',')
   File "/usr/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 3092, in plot
     genGraph.generer_graph(chemin=self.chemin, y_low_min=self.y_low_min, y_low_max=self.y_low_max, y_high_min=self.y_high_min, y_high_max=self.y_high_max, format_graph=self.format_graph)
   File "/usr/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 3092, in plot
     ax = gca()
     ax = gca()
   File "/home/pi/Desktop/interfacebancdetest_dev/genGraph.py", line 48, in generer_graph
   File "/usr/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 828, in gca
   File "/usr/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 828, in gca
     ax =  gcf().gca(**kwargs)
     plt.plot(abcisse, ordonnee, marker=',')
     ax =  gcf().gca(**kwargs)
   File "/usr/lib/python2.7/dist-packages/matplotlib/figure.py", line 1268, in gca
   File "/usr/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 3092, in plot
   File "/usr/lib/python2.7/dist-packages/matplotlib/figure.py", line 1268, in gca
     return self.add_subplot(1, 1, 1, **kwargs)
     ax = gca()
     return self.add_subplot(1, 1, 1, **kwargs)
   File "/usr/lib/python2.7/dist-packages/matplotlib/figure.py", line 960, in add_subplot
   File "/usr/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 828, in gca
   File "/usr/lib/python2.7/dist-packages/matplotlib/figure.py", line 960, in add_subplot
     self._axstack.add(key, a)
     ax =  gcf().gca(**kwargs)
     self._axstack.add(key, a)
   File "/usr/lib/python2.7/dist-packages/matplotlib/figure.py", line 124, in add
   File "/usr/lib/python2.7/dist-packages/matplotlib/figure.py", line 1268, in gca
   File "/usr/lib/python2.7/dist-packages/matplotlib/figure.py", line 124, in add
     Stack.remove(self, (key, a_existing))
     return self.add_subplot(1, 1, 1, **kwargs)
     Stack.remove(self, (key, a_existing))
   File "/usr/lib/python2.7/dist-packages/matplotlib/cbook.py", line 1383, in remove
   File "/usr/lib/python2.7/dist-packages/matplotlib/figure.py", line 960, in add_subplot
   File "/usr/lib/python2.7/dist-packages/matplotlib/cbook.py", line 1383, in remove
     raise ValueError('Unknown element o')
     self._axstack.add(key, a)
     raise ValueError('Unknown element o')
 ValueError: Unknown element o
   File "/usr/lib/python2.7/dist-packages/matplotlib/figure.py", line 124, in add

 ValueError: Unknown element o
     Stack.remove(self, (key, a_existing))

   File "/usr/lib/python2.7/dist-packages/matplotlib/cbook.py", line 1383, in remove
     raise ValueError('Unknown element o')
 ValueError: Unknown element o

[INFO   ] [Base        ] Leaving application in progress...

As I understand it, it could be a problem of shared intern ressources between the differents threads, but I relly don't know how to solve this one....
My code : 
if(passerelle.graph1.flagGraph):
            t1=Thread_graph(chemin=chemin1, y_low_min=passerelle.graph1.y_low_min, y_low_max=passerelle.graph1.y_low_max, y_high_min=passerelle.graph1.y_high_min, y_high_max=passerelle.graph1.y_high_max, format_graph=passerelle.graph1.setTaille)
            t2=Thread_graph(chemin=chemin2, y_low_min=passerelle.graph1.y_low_min, y_low_max=passerelle.graph1.y_low_max, y_high_min=passerelle.graph1.y_high_min, y_high_max=passerelle.graph1.y_high_max, format_graph=passerelle.graph1.setTaille)
            t3=Thread_graph(chemin=chemin3, y_low_min=passerelle.graph1.y_low_min, y_low_max=passerelle.graph1.y_low_max, y_high_min=passerelle.graph1.y_high_min, y_high_max=passerelle.graph1.y_high_max, format_graph=passerelle.graph1.setTaille)
            t4=Thread_graph(chemin=chemin4, y_low_min=passerelle.graph1.y_low_min, y_low_max=passerelle.graph1.y_low_max, y_high_min=passerelle.graph1.y_high_min, y_high_max=passerelle.graph1.y_high_max, format_graph=passerelle.graph1.setTaille)
            try:
                t1.start()
                #genGraph.generer_graph(chemin=chemin1, y_low_min=passerelle.graph1.y_low_min, y_low_max=passerelle.graph1.y_low_max, y_high_min=passerelle.graph1.y_high_min, y_high_max=passerelle.graph1.y_high_max, format_graph=passerelle.graph1.setTaille)

            except AttributeError:
                Logger.warning('fermerBanc: impossible de generer un graph 1, fichier vide')

            try:
                t2.start()
                #genGraph.generer_graph(chemin=chemin2, y_low_min=passerelle.graph1.y_low_min, y_low_max=passerelle.graph1.y_low_max, y_high_min=passerelle.graph1.y_high_min, y_high_max=passerelle.graph1.y_high_max, format_graph=passerelle.graph1.setTaille)

            except AttributeError:
                Logger.warning('fermerBanc: impossible de generer un graph 2, fichier vide')

            try:
                t3.start()
                #genGraph.generer_graph(chemin=chemin3, y_low_min=passerelle.graph1.y_low_min, y_low_max=passerelle.graph1.y_low_max, y_high_min=passerelle.graph1.y_high_min, y_high_max=passerelle.graph1.y_high_max, format_graph=passerelle.graph1.setTaille)

            except AttributeError:
                Logger.warning('fermerBanc: impossible de generer un graph 3, fichier vide')

            try:
                t4.start()
                #genGraph.generer_graph(chemin=chemin4, y_low_min=passerelle.graph1.y_low_min, y_low_max=passerelle.graph1.y_low_max, y_high_min=passerelle.graph1.y_high_min, y_high_max=passerelle.graph1.y_high_max, format_graph=passerelle.graph1.setTaille)

            except AttributeError:
                Logger.warning('fermerBanc: impossible de generer un graph 4, fichier vide')

            t1.join()
            t2.join()
            t3.join()
            t4.join()

My class
class Thread_graph(Thread):
    """Permet le multi threading de generation de graph"""
    def __init__(self, chemin, y_low_min, y_low_max, y_high_min, y_high_max, format_graph):
        Thread.__init__(self)
        self.chemin = chemin
        self.y_low_min=y_low_min
        self.y_low_max=y_low_max
        self.y_high_min=y_high_min
        self.y_high_max=y_high_max
        self.format_graph=format_graph

    def run(self):
        """code à executer pendand l'executiond u thread"""
        genGraph.generer_graph(chemin=self.chemin, y_low_min=self.y_low_min, y_low_max=self.y_low_max, y_high_min=self.y_high_min, y_high_max=self.y_high_max, format_graph=self.format_graph)

A:

Found that due to back end conditions, matplotlib is not "thread safe". So i used process instead (And since my fonction needed several arguments, I had to modify it to pass a list of objects instead)
process doc

