use another strategy to avoid matplotlib backend error

Former-commit-id: d20b0efb39ae273a99bfdfa19d9fe216afe2ec7b

ippisite/ippidb/management/commands/pca.py

@@ -6,7 +6,6 @@ import base64
 from django.core.management import BaseCommand, CommandError
 from django.forms.models import model_to_dict 
 import matplotlib.pyplot as plt
-matplotlib.use('Agg')
 import pandas as pd
 import numpy as np
 from sklearn.decomposition import PCA
@@ -55,6 +54,7 @@ class Command(BaseCommand):
         pca = PCA(n_components=2)
         principal_components = pca.fit_transform(x)
         # compute correlation circle
+        plt.switch_backend('Agg')
         variance_ratio = pd.Series(pca.explained_variance_ratio_)
         coef = np.transpose(pca.components_)
         cols = ['PC-'+str(x) for x in range(len(variance_ratio))]