Update asciigraph_test.go

Add test case for all 0 values in data.

Author: guptarohit

asciigraph_test.go

@@ -15,6 +15,10 @@ func TestPlot(t *testing.T) {
 			[]float64{1, 1, 1, 1, 1},
 			nil,
 			` 1.00 ┼──── `},
+		{
+			[]float64{0, 0, 0, 0, 0},
+			nil,
+			` 0.00 ┼──── `},
 		{
 			[]float64{2, 1, 1, 2, -2, 5, 7, 11, 3, 7, 1},
 			nil,