You may be able get all of the labels to appear if you use las=2 inside the plot() call. This argument and the others mentioned below are described in ?par which sets the graphical parameters for plotting devices. That rotates the text 90 degrees. Otherwise, you will need to use xaxt="n" (to suppress ticks and labels) and then put the labels in with a separate call to axis(1, at= <some numerical vector>, labels=<some character vector>).
# midpts <- barplot( ... ) # assign result to named object
axis(1, at = midpts, labels=names(DD), cex.axis=0.7) # shrinks axis labels
Another method is to first collect the midpoints and then use text() with xpd=TRUE to allow text to appear outside the plot area and srt be some angle for text rotation as named arguments to control the degree of text rotation:
text(x=midpts, y=-2, names(DD), cex=0.8, srt=45, xpd=TRUE)
The y-value needs to be chosen using the coordinates in the plotted area.
Copying a useful comment: For future readers who don’t know what these arguments do: las=2 rotates the labels counterclockwise by 90 degrees. furthermore, if you need to reduce the font you can use cex.names=.5 to shrink the size down