fix: wg.Done for all goroutines expected but not created

whenever at least 1 of the n expected activities failed to scrape,
the pipeline stage of genImg would block because it created a
wg of size n.

To remediate this, a counter of the number of created go routines
is kept by the genImg pipeline stage. Once the input channel is
closed (no more graphs to turn into images), the stage wg.Done's
n minus the number of created goroutines.

main.go

@@ -201,26 +201,37 @@ func genImg(in <-chan graph, size int) <-chan activityImage {
 	if err != nil {
 		log.Fatal(err)
 	}
+	labelColor := color.RGBA{88, 96, 105, 0xff}
+	valueColor := color.RGBA{149, 157, 165, 0xff}
+	axisColor := color.RGBA{108, 178, 103, 0xff}
+	polyColor := color.RGBA{123, 201, 111, 0xff}
 
 	var out = make(chan activityImage, size)
 	var wg sync.WaitGroup
 	wg.Add(size)
+	activeGoRoutines := 0
 	go func() {
 		for g := range in {
+			activeGoRoutines++
 			go func(g graph) {
 				defer wg.Done()
 				s := style{
-					LabelColor:   color.RGBA{88, 96, 105, 0xff},
-					ValueColor:   color.RGBA{149, 157, 165, 0xff},
-					AxisColor:    color.RGBA{108, 178, 103, 0xff},
-					PolyColor:    color.RGBA{123, 201, 111, 0xff},
+					MarkerRadius: 6,
+					LabelColor:   labelColor,
+					ValueColor:   valueColor,
+					AxisColor:    axisColor,
+					PolyColor:    polyColor,
 					LabelFont:    truetype.NewFace(font, &truetype.Options{Size: 24}),
 					ValueFont:    truetype.NewFace(font, &truetype.Options{Size: 22}),
-					MarkerRadius: 6,
 				}
 				out <- activityImage{img(g, s), g.Data.Year}
 			}(g)
 		}
+		// when input channel is closed, reduce the waitgroup counter
+		// by the number of goroutines that were expected but not created
+		for i := 0; i < size-activeGoRoutines; i++ {
+			wg.Done()
+		}
 	}()
 	go func() {
 		wg.Wait()