fixed bug with extend and append

spb/backends/base_backend.py

@@ -617,6 +617,8 @@ def append(self, arg):
                 self.legend = True
         else:
             raise TypeError("Must specify element of plot to append.")
+        # recreate renderers
+        self._create_renderers()
 
     def extend(self, arg):
         """Adds all series from another plot.
@@ -664,3 +666,5 @@ def extend(self, arg):
         # auto legend
         if len(self._series) > 1:
             self.legend = True
+        # recreate renderers
+        self._create_renderers()

tests/backends/test_base_backend.py

@@ -1,5 +1,5 @@
 from spb.backends.base_backend import Plot
-from spb.series import BaseSeries
+from spb.series import BaseSeries, HVLineSeries
 from pytest import raises
 import matplotlib
 import matplotlib.pyplot as plt
@@ -178,3 +178,27 @@ def test_xaxis_inverted():
         ylim=(-1.25, 1.25), backend=MB, use_latex=False, show=False, n=10
     )
     p.backend.draw()
+
+
+def test_number_of_renderers():
+    # verify that once `extend` or `append` is executed, the number of
+    # renderers will be equal to the new number of series
+
+    x = symbols("x")
+    hor = HVLineSeries(0, True, show_in_legend=False)
+    ver1 = HVLineSeries(0, False, show_in_legend=False)
+    ver2 = HVLineSeries(1, False, show_in_legend=False)
+
+    def do_test(B):
+        p = plot(sin(x), (x, -5, 5), backend=B, show=False, n=5)
+        assert len(p.series) == len(p.renderers) == 1
+
+        p.extend([hor, ver1])
+        assert len(p.series) == len(p.renderers) == 3
+
+        p.append(ver2)
+        assert len(p.series) == len(p.renderers) == 4
+
+    do_test(MB)
+    do_test(PB)
+    do_test(BB)