Plot percentages on y-axis

library(scales)
dates <-  1:100
returns <- runif(100)
yticks_val <- pretty_breaks(n=5)(returns)
plot(dates, returns, yaxt="n")
axis(2, at=yticks_val, lab=percent(yticks_val))

Highlights:

1. No need to explicitly add "%"
2. Manually fix the number of y-ticks to be consistent with further plots. Here I chose 5.

Combining two answers together @rengis @vladiim

Here is one way using las=TRUE to turn the labels on the y-axis and axis() for the new y-axis with adjusted labels.

dates <-  1:10
returns <- runif(10)

plot(dates, returns, yaxt="n")
axis(2, at=pretty(returns), lab=pretty(returns) * 100, las=TRUE)

If you use ggplot you can use the scales package.

library(scales)
plot + scale_y_continuous(labels = percent)