handle NaN values in series

fixes #21

asciigraph.go

@@ -108,8 +108,29 @@ func Plot(series []float64, options ...Option) string {
 	plot[rows-y0][config.Offset-1] = "┼" // first value
 
 	for x := 0; x < len(series)-1; x++ { // plot the line
-		y0 = int(round(series[x+0]*ratio) - float64(intmin2))
-		y1 = int(round(series[x+1]*ratio) - float64(intmin2))
+
+		d0 := series[x]
+		d1 := series[x+1]
+
+		if math.IsNaN(d0) && math.IsNaN(d1) {
+			continue
+		}
+
+		if math.IsNaN(d1) && !math.IsNaN(d0) {
+			y0 = int(round(d0*ratio) - float64(intmin2))
+			plot[rows-y0][x+config.Offset] = "╴"
+			continue
+		}
+
+		if math.IsNaN(d0) && !math.IsNaN(d1) {
+			y1 = int(round(d1*ratio) - float64(intmin2))
+			plot[rows-y1][x+config.Offset] = "╶"
+			continue
+		}
+
+		y0 = int(round(d0*ratio) - float64(intmin2))
+		y1 = int(round(d1*ratio) - float64(intmin2))
+
 		if y0 == y1 {
 			plot[rows-y0][x+config.Offset] = "─"
 		} else {

asciigraph_test.go

@@ -2,6 +2,7 @@ package asciigraph
 
 import (
 	"fmt"
+	"math"
 	"testing"
 )
 
@@ -187,6 +188,31 @@ func TestPlot(t *testing.T) {
  38.46 ┤                                                     ││ │││ 
  37.46 ┤                                                     ╰╯ │││ 
  36.45 ┤                                                        ╰╯╰ `},
+		{
+			[]float64{1, 1, math.NaN(), 1, 1},
+			nil,
+			` 1.00 ┼─╴╶─ `},
+		{
+			[]float64{0, 0, 1, 1, math.NaN(), math.NaN(), 3, 3, 4},
+			nil,
+			` 4.00 ┼       ╭ 
+ 3.00 ┤     ╶─╯ 
+ 2.00 ┤         
+ 1.00 ┤ ╭─╴     
+ 0.00 ┼─╯       `},
+		{
+			[]float64{.1, .2, .3, math.NaN(), .5, .6, .7, math.NaN(), math.NaN(), .9, 1},
+			nil,
+			` 1.00 ┤         ╭ 
+ 0.90 ┤        ╶╯ 
+ 0.80 ┤           
+ 0.70 ┤     ╭╴    
+ 0.60 ┤    ╭╯     
+ 0.50 ┤   ╶╯      
+ 0.40 ┤           
+ 0.30 ┤ ╭╴        
+ 0.20 ┤╭╯         
+ 0.10 ┼╯          `},
 	}
 
 	for i := range cases {