unification of sample and test coding styles will improve visibility (#67)

* "Style{Show: true}" to "StyleShow()"
* "Box{IsSet: true}" to "BoxZero"

annotation_series_test.go

@@ -5,6 +5,7 @@ import (
 	"testing"
 
 	"github.com/blend/go-sdk/assert"
+
 	"github.com/wcharczuk/go-chart/drawing"
 )
 
@@ -12,9 +13,7 @@ func TestAnnotationSeriesMeasure(t *testing.T) {
 	assert := assert.New(t)
 
 	as := AnnotationSeries{
-		Style: Style{
-			Show: true,
-		},
+		Style: StyleShow(),
 		Annotations: []Value2{
 			{XValue: 1.0, YValue: 1.0, Label: "1.0"},
 			{XValue: 2.0, YValue: 2.0, Label: "2.0"},