Sanity check dash arguments (#910)

* Sanity check dash arguments.

Fixes #909.

* Fix dash sanity test.

Author: corranwebster

kiva/celiagg.py

@@ -280,6 +280,15 @@ def set_line_dash(self, lengths, phase=0):
         """
         if lengths is not None:
             count = len(lengths)
+            if count % 2 == 1:
+                raise ValueError(
+                    "Array of lengths should have an even number of values, "
+                    f"got {lengths!r}"
+                )
+            if any(length <= 0 for length in lengths):
+                raise ValueError(
+                    f"All length values should be positive, got {lengths!r}"
+                )
             lengths = np.array(lengths).reshape(count // 2, 2)
         else:
             lengths = []

kiva/tests/test_celiagg_drawing.py

@@ -14,6 +14,7 @@
 
 
 class TestCeliaggDrawing(DrawingImageTester, unittest.TestCase):
+
     def create_graphics_context(self, width, height, pixel_scale):
         return GraphicsContext((width, height), base_pixel_scale=pixel_scale)
 
@@ -37,3 +38,12 @@ def test_ipython_repr_png(self):
         with open(filename, 'wb') as fp:
             fp.write(stream)
         self.assertImageSavedWithContent(filename)
+
+    def test_set_line_dash_odd(self):
+        with self.assertRaises(ValueError):
+            self.gc.set_line_dash([1.0, 2.0, 3.0])
+
+    def test_set_line_dash_positive(self):
+        # regression test for #909
+        with self.assertRaises(ValueError):
+            self.gc.set_line_dash([0.0, 0.0])