Here comes my solutions.
x <- c(33.75, 33.05, 34, 33.81, 33.46, 34.02, 33.68, 33.27, 33.49, 33.20, 33.62, 33.00, 33.54, 33.12, 33.84)
# clear space to start painting
dev.off()
# 1. solve the background color
qcc.options(bg.margin="white")
# 2. make plot with blank axis and set tick labels afterwards
par(xaxt="n") #force plot to hide x-axis tick
q <-ewma(x, lambda=0.2, nsigmas=2.7, add.stats=F)
par(xaxt="s") # allow to have new x-axis tick
axis(1, at=c(1,5,15),label=c(1,5,15)) # create your own ticks
# 3. add legends anywhere you want, use expression() to type Greek letters
legend(10,33.8,expression("s = 5" ~lambda ~ " = 0.133" , "s = 6" ~lambda ~ " = 0.117" ),lty=1:2,col=1:2,lwd=1:2)