Don't panic on empty plot data (#1090)

Author: rohansingh

render/plot.go

@@ -29,25 +29,27 @@ var FillDampFactor uint8 = 0x55
 //
 // EXAMPLE BEGIN
 // render.Plot(
-//   data = [
-//     (0, 3.35),
-//     (1, 2.15),
-//     (2, 2.37),
-//     (3, -0.31),
-//     (4, -3.53),
-//     (5, 1.31),
-//     (6, -1.3),
-//     (7, 4.60),
-//     (8, 3.33),
-//     (9, 5.92),
-//   ],
-//   width = 64,
-//   height = 32,
-//   color = "#0f0",
-//   color_inverted = "#f00",
-//   x_lim = (0, 9),
-//   y_lim = (-5, 7),
-//   fill = True,
+//
+//	data = [
+//	  (0, 3.35),
+//	  (1, 2.15),
+//	  (2, 2.37),
+//	  (3, -0.31),
+//	  (4, -3.53),
+//	  (5, 1.31),
+//	  (6, -1.3),
+//	  (7, 4.60),
+//	  (8, 3.33),
+//	  (9, 5.92),
+//	],
+//	width = 64,
+//	height = 32,
+//	color = "#0f0",
+//	color_inverted = "#f00",
+//	x_lim = (0, 9),
+//	y_lim = (-5, 7),
+//	fill = True,
+//
 // ),
 // EXAMPLE END
 type Plot struct {
@@ -95,6 +97,10 @@ func (p *Plot) computeLimits() (float64, float64, float64, float64) {
 	}
 
 	// Otherwise we'll need min/max of X and Y
+	if len(p.Data) == 0 {
+		return 0, 1, 0, 1
+	}
+
 	pt := p.Data[0]
 	minX, maxX, minY, maxY := pt[0], pt[0], pt[1], pt[1]
 	for i := 1; i < len(p.Data); i++ {

render/plot_test.go

@@ -301,6 +301,37 @@ func TestPlotJaggedLine(t *testing.T) {
 
 }
 
+func TestPlotEmpty(t *testing.T) {
+	ic := ImageChecker{
+		Palette: map[string]color.RGBA{
+			"1": {0xff, 0xff, 0xff, 0xff},
+			".": {0, 0, 0, 0},
+		},
+	}
+
+	p := Plot{
+		Width:  10,
+		Height: 10,
+		Data:   [][2]float64{},
+		XLim:   Empty,
+		YLim:   Empty,
+	}
+
+	assert.Equal(t, nil, ic.Check([]string{
+		"..........",
+		"..........",
+		"..........",
+		"..........",
+		"..........",
+		"..........",
+		"..........",
+		"..........",
+		"..........",
+		"..........",
+	}, PaintWidget(p, image.Rect(0, 0, 100, 100), 0)))
+
+}
+
 func TestPlotFewPoints(t *testing.T) {
 	ic := ImageChecker{
 		Palette: map[string]color.RGBA{