Add alpha option to ggviolin

[florianduclot]

Add option to add alpha transparency to ggviolin().

Tested with default example for ggviolin plot:

# Load data
data("ToothGrowth")
df <- ToothGrowth
head(df, 4)

my_comparisons <- list( c("0.5", "1"), c("1", "2"), c("0.5", "2") )


# Violin plots with box plots inside
# :::::::::::::::::::::::::::::::::::::::::::::::::::
# Change fill color by groups: dose
# add boxplot with white fill color
ggviolin(df, x = "dose", y = "len", fill = "dose", alpha = 0.5,
         palette = c("#00AFBB", "#E7B800", "#FC4E07"),
         add = "boxplot", add.params = list(fill = "white"))+
  stat_compare_means(comparisons = my_comparisons, label = "p.signif")+ # Add significance levels
  stat_compare_means(label.y = 50)           

[florianduclot]

I finally added mention of the alpha option to the ggviolin manpage.
Sorry for taking so long for that, and thanks for having a look at this PR!

[kassambara]

Thank you for your contribution