[R] how to make the figure more clearly

[Abelian]

Dear all
when i draw some figures by R, i try to use "jpeg" to display my
result.
However, it isn't still clear as i want.
Especially when i compare the origial figures by R and the ".jpeg"
file

the code is follwed
jpeg(filename=....,width = 1200, height = 800, pointsize = 14)
..............
dev.off()

so, how can i make the figure more clearly?
Or should I change the other way to show off my figures?
Sincerely