Fix https://github.com/gizak/termui/issues/9

chart.go

@@ -4,7 +4,10 @@
 
 package termui
 
-import "fmt"
+import (
+	"fmt"
+	"math"
+)
 
 // only 16 possible combinations, why bother
 var braillePatterns = map[[2]int]rune{
@@ -77,6 +80,8 @@ func NewLineChart() *LineChart {
 	lc.DotStyle = '•'
 	lc.axisXLebelGap = 2
 	lc.axisYLebelGap = 1
+	lc.bottomValue = math.Inf(1)
+	lc.topValue = math.Inf(-1)
 	return lc
 }