Added test to prevent regression fixed by recent patch

tests/test_plot.py

@@ -0,0 +1,20 @@
+
+import unittest
+
+class PlotTester(unittest.TestCase):
+	def test_index_error_2010_04(self):
+		"""
+		Reported by Jean Schurger
+		a 'IndexError: tuple index out of range' when there are only two
+		values returned by float_range (in the case there are only two
+		different 'y' values in the data) and 'scale_y_integers == True'.
+		
+		Credit to Jean for the test code as well.
+		"""
+		from svg.charts.plot import Plot
+		g = Plot(dict(scale_y_integers = True))
+		g.add_data(dict(data=[1, 0, 2, 1], title='foo'))
+		res = g.burn()
+
+if __name__ == '__main__':
+	unittest.main()