Fix #2590: Created new axes for plot of persistence_length. (#2591)

* Fixes #2590
* Created new axes for plot of persistence_length if ax is not passed in as an argument (analysis/polymer.py)
* Added test (analysis/test_persistencelength.py)

package/CHANGELOG

@@ -111,6 +111,7 @@ Enhancements
   * Improve the distance search in water bridge analysis with capped_distance (PR #2480)
   * Added weights_groupselections option in RMSD for mapping custom weights 
     to groupselections (PR #2610, Issue #2429)
+  * PersistenceLength.plot() now create new axes if current axes not provided (Issue #2590)
 
 Changes
   * Removed `details` from `ClusteringMethod`s (Issue #2575, PR #2620)

package/MDAnalysis/analysis/polymer.py

@@ -268,7 +268,7 @@ def plot(self, ax=None):
         """
         import matplotlib.pyplot as plt
         if ax is None:
-            ax = plt.gca()
+            fig, ax = plt.subplots()
         ax.plot(self.x, self.results, 'ro', label='Result')
         ax.plot(self.x, self.fit, label='Fit')
         ax.set_xlabel(r'x')

testsuite/MDAnalysisTests/analysis/test_persistencelength.py

@@ -92,6 +92,11 @@ def test_plot_with_ax(self, p_run):
         ax2 = p_run.plot(ax=ax)
 
         assert ax2 is ax
+
+    def test_current_axes(self, p_run):
+        fig, ax = plt.subplots()
+        ax2 = p_run.plot(ax=None)
+        assert ax2 is not ax
 
 
 class TestFitExponential(object):