Display the names of specific elements in a plot

I have a volcano plot as shown in

http://i.stack.imgu开发者_运维知识库r.com/UreCs.jpg

I need to display the names of ONLY the elements which are shown in red color out of 19000 elements

How can I do that?

Also is there anyways I could represent those elements in the red color in different sizes based on their fold change and p-values?

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You can label elements with the text function. You could change the size of elements with the cex argument to plot, for instance

plot(1:10, cex=1:10*0.2)
text(c(1,2)+0.4, c(1,2), c("P1", "P2"))

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Jana,

You can annotate your plots with text. You can change many of the different elements of your plot by passing additional parameters in your call to plot. Most of these parameters are discussed in detail on the ?par page. Finally, you can specify logic in your calls to plot based on something of interest. Here's a quick example:

df <- data.frame(x = rnorm(100), y = rnorm(100))

with(df, plot(x,y
    , cex = ifelse(abs(x) > 2, .75, 1.5)     #Size
    , col = ifelse(abs(x) > 2 | abs(y) > 2, "red", "blue")     #Color
    , pch = ifelse(abs(x) > 2 | abs(y) > 2, 4, 6)     #Type of point
    )
    )
text(2, -3, "This is text on my plot")