<p>In a lot of biological oriented paper, one often finds
figure like this:</p>

<p><img src='http://s15.postimg.org/6zyqpmbqz/test.png' alt='enter image description here' /></p>

<p>What is the program usually people used for drawing that?
If R, how do they achieve the horizontal marker and asterisks?</p>

<p>Not the most elegant solution, but you can use the regular R utilities for drawing lines and asterisks. You can capture the center positions of the bars by assigning the barplot call to a variable. The lines() and text() functions take x and y coordinates of where to draw things:</p>

<pre><code># capture x coordinates of bars
x &lt;- barplot(c(1,4,3,6,5))
# create the y coordinate of the line
y &lt;- 4.5
# set an offset for tick lengths
offset &lt;- 0.2
# draw first horizontal line
lines(x[1:2],c(y, y))
# draw ticks
lines(x[c(1,1)],c(y, y-offset))
lines(x[c(2,2)],c(y, y-offset))
# draw asterics
text(x[1]+((x[2]-x[1])/2),y+offset,"**")
</code></pre>

<p>You simply have to do some guessing at the offsets.</p>
