When saving a ggbetweenstats graph to a PDF file through ggsave, if violinplot is removed as indicated in the documentation

# to remove violin plot
violin.args = list(width = 0, linewidth = 0)

a vertical black line appears through the whole range of values.

To solve the issue, it is sufficient to specify the colour parameter to NA

# to remove violin plot
violin.args = list(width = 0, linewidth = 0, colour = NA)

Additionally, if the graph is saved in PNG, the greek character is not rendered properly.