Fix https://github.com/gizak/termui/issues/75

Negative numbers are suppressed as zeros. The height of each data point
is based off zero, so it will be confusing when it comes to negative
numbers. Simply treat negative values data points as zeroes.

sparkline.go

@@ -4,9 +4,7 @@
 
 package termui
 
-import "math"
-
-// Sparkline is like: ▅▆▂▂▅▇▂▂▃▆▆▆▅▃
+// Sparkline is like: ▅▆▂▂▅▇▂▂▃▆▆▆▅▃. The data points should be non-negative integers.
 /*
   data := []int{4, 2, 1, 6, 3, 9, 1, 4, 2, 15, 14, 9, 8, 6, 10, 13, 15, 12, 10, 5, 3, 6, 1}
   spl := termui.NewSparkline()
@@ -84,14 +82,18 @@ func (sl *Sparklines) update() {
 	for i := 0; i < sl.displayLines; i++ {
 		data := sl.Lines[i].Data
 
-		max := math.MinInt32
+		max := 0
 		for _, v := range data {
 			if max < v {
 				max = v
 			}
 		}
 		sl.Lines[i].max = max
-		sl.Lines[i].scale = float32(8*sl.Lines[i].Height) / float32(max)
+		if max != 0 {
+			sl.Lines[i].scale = float32(8*sl.Lines[i].Height) / float32(max)
+		} else { // when all negative
+			sl.Lines[i].scale = 0
+		}
 	}
 }
 
@@ -127,7 +129,12 @@ func (sl *Sparklines) Buffer() Buffer {
 		}
 
 		for j, v := range data {
+			// display height of the data point, zero when data is negative
 			h := int(float32(v)*l.scale + 0.5)
+			if v < 0 {
+				h = 0
+			}
+
 			barCnt := h / 8
 			barMod := h % 8
 			for jj := 0; jj < barCnt; jj++ {