Reject unevenly spaced x/y in create_streamline when steps shrink

validate_streamline compared each step with the first one without taking
the absolute value, so only steps larger than the first were caught and
x such as [0, 2, 3, 3.5] passed as evenly spaced.

Author: raashish1601

plotly/figure_factory/_streamline.py

@@ -23,12 +23,12 @@ def validate_streamline(x, y):
     if np is False:
         raise ImportError("FigureFactory.create_streamline requires numpy")
     for index in range(len(x) - 1):
-        if ((x[index + 1] - x[index]) - (x[1] - x[0])) > 0.0001:
+        if abs((x[index + 1] - x[index]) - (x[1] - x[0])) > 0.0001:
             raise exceptions.PlotlyError(
                 "x must be a 1 dimensional, evenly spaced array"
             )
     for index in range(len(y) - 1):
-        if ((y[index + 1] - y[index]) - (y[1] - y[0])) > 0.0001:
+        if abs((y[index + 1] - y[index]) - (y[1] - y[0])) > 0.0001:
             raise exceptions.PlotlyError(
                 "y must be a 1 dimensional, evenly spaced array"
             )

tests/test_optional/test_figure_factory/test_figure_factory.py

@@ -64,6 +64,28 @@ def test_uneven_y(self):
         }
         self.assertRaises(PlotlyError, ff.create_streamline, **kwargs)
 
+    def test_uneven_x_shrinking_steps(self):
+        # check for PlotlyError if x steps get smaller than the first one
+
+        kwargs = {
+            "x": [0, 2, 3, 3.5],
+            "y": [0, 1, 2, 3],
+            "u": [[1, 1, 1, 1]] * 4,
+            "v": [[1, 1, 1, 1]] * 4,
+        }
+        self.assertRaises(PlotlyError, ff.create_streamline, **kwargs)
+
+    def test_uneven_y_shrinking_steps(self):
+        # check for PlotlyError if y steps get smaller than the first one
+
+        kwargs = {
+            "x": [0, 1, 2, 3],
+            "y": [0, 2, 3, 3.5],
+            "u": [[1, 1, 1, 1]] * 4,
+            "v": [[1, 1, 1, 1]] * 4,
+        }
+        self.assertRaises(PlotlyError, ff.create_streamline, **kwargs)
+
     def test_unequal_length_xy(self):
         # check for PlotlyError if u and v are not the same length