The behavior you are seeing is an intentional behavior with R. You need to surround
http://cran.r-project.org/doc/manuals/r-release/R-ints.html#Autoprinting
ggplot() + ggplot() + geom_line(data = my_data, my_data$Price)
print(ggplot() + ggplot() + geom_line(data = my_data, my_data$Price))
http://cran.r-project.org/doc/manuals/r-release/R-ints.html#Autoprinting