To select columns to plot, I added 2 lines to Vincent Zoonekynd's answer:
#convert to tall/long format(from wide format)
col_plot = c("A","B")
dlong <- melt(d[,c("Xax", col_plot)], id.vars="Xax")
#"value" and "variable" are default output column names of melt()
ggplot(dlong, aes(Xax,value, col=variable)) +
geom_point() +
geom_smooth()
Google "tidy data" to know more about tall(or long)/wide format.