This closes #1910, fix a v2.8.1 regression bug introduced by commit 866f3086cd

- Fix spark lines duplicate when creating spark lines on multiple sheets

sparkline.go

@@ -18,9 +18,10 @@ import (
 	"strings"
 )
 
-// sparklineGroupPresets defined the list of sparkline group to create
-// x14:sparklineGroups element by given sparkline style ID.
-var sparklineGroupPresets = []*xlsxX14SparklineGroup{
+// getSparklineGroupPresets returns the preset list of sparkline group to create
+// x14:sparklineGroups element.
+func getSparklineGroupPresets() []*xlsxX14SparklineGroup {
+	return []*xlsxX14SparklineGroup{
 		{
 			ColorSeries:   &xlsxColor{Theme: intPtr(4), Tint: -0.499984740745262},
 			ColorNegative: &xlsxColor{Theme: intPtr(5)},
@@ -355,6 +356,7 @@ var sparklineGroupPresets = []*xlsxX14SparklineGroup{
 			ColorLow:      &xlsxColor{Theme: intPtr(7)},
 		}, // 36
 	}
+}
 
 // AddSparkline provides a function to add sparklines to the worksheet by
 // given formatting options. Sparklines are small charts that fit in a single
@@ -412,7 +414,7 @@ func (f *File) AddSparkline(sheet string, opts *SparklineOptions) error {
 		}
 		sparkType = specifiedSparkTypes
 	}
-	group = sparklineGroupPresets[opts.Style]
+	group = getSparklineGroupPresets()[opts.Style]
 	group.Type = sparkType
 	group.ColorAxis = &xlsxColor{RGB: "FF000000"}
 	group.DisplayEmptyCellsAs = "gap"