fix wrong dendrogram leaves determination bug (#1186)

* fix wrong leaves determination bug
* add test_dendrogram_ticklabels and comments about why/how to fix the dendrogram bugs.

Author: 444thLiao

Diff for: plotly/figure_factory/_dendrogram.py

@@ -141,8 +141,20 @@ def __init__(self, X, orientation='bottom', labels=None, colorscale=None,
             if yvals_flat[i] == 0.0 and xvals_flat[i] not in self.zero_vals:
                 self.zero_vals.append(xvals_flat[i])
 
-        self.zero_vals.sort()
+        if len(self.zero_vals) > len(yvals) + 1:
+            # If the length of zero_vals is larger than the length of yvals,
+            # it means that there are wrong vals because of the identicial samples.
+            # Three and more identicial samples will make the yvals of spliting center into 0 and it will \
+            # accidentally take it as leaves.
+            l_border = int(min(self.zero_vals))
+            r_border = int(max(self.zero_vals))
+            correct_leaves_pos = range(l_border,
+                                       r_border + 1,
+                                       int((r_border - l_border) / len(yvals)))
+            # Regenerating the leaves pos from the self.zero_vals with equally intervals.
+            self.zero_vals = [v for v in correct_leaves_pos]
 
+        self.zero_vals.sort()
         self.layout = self.set_figure_layout(width, height)
         self.data = dd_traces
 

Diff for: plotly/tests/test_optional/test_figure_factory/test_figure_factory.py

@@ -777,6 +777,15 @@ def test_dendrogram_colorscale(self):
         self.assert_fig_equal(dendro['data'][1], expected_dendro['data'][1])
         self.assert_fig_equal(dendro['data'][2], expected_dendro['data'][2])
 
+    def test_dendrogram_ticklabels(self):
+        X = np.array([[1, 2, 3, 4], [1, 2, 3, 4], [1, 3, 5, 6], [1, 4, 2, 3]])
+        dendro = ff.create_dendrogram(X=X)
+
+        expected_ticktext = ['2', '3', '0', '1']
+        expected_tickvals = [5, 15, 25, 35]
+
+        self.assertEqual(len(dendro.layout.xaxis.ticktext), 4)
+        self.assertEqual(len(dendro.layout.xaxis.tickvals), 4)
 
 class TestTrisurf(NumpyTestUtilsMixin, TestCase):