drawing a circle of radius R around a point

Now if I have many points (in a txt file where each line is x y) and I want to draw a circles which different specified radius for each point. Should I repeat the command "set object i circle at Xi,Yi size first Ri fc rgb "navy"" for each point i ?!

Answer: No! Plotting with circles was available in gnuplot V4.4 (2010).

"Circles.dat"

1 1 0.1
2 2 0.2
3 3 0.3
4 4 0.4
5 5 0.5
6 6 0.6

Code:

plot "Circles.dat" u 1:2:3:1 w circles lc var notitle

Result: (created with gnuplot 4.4)

Working gnuplot script:

# tell gnuplot where we want to look at
set xrange [0:1]
set yrange [0:1]

# make a square plot
set size square

# create a black circle at center (0.5, 0.5) with radius 0.5
set object 1 circle front at 0.5,0.5 size 0.5 fillcolor rgb "black" lw 1

f(x) = x # we need to plot at lest one function
plot f(x) # show the stuff

If you don't want to graph a circle, you can use the set object circle command. You use it like this, for example:

set object X circle at axis 0,0 size scr 0.1 fc rgb "navy"

This will draw a navy blue circle at the origin with a radius of 0.1 of the screen (canvas) size. Note that when you specify a position/radius for the circle you have to specify which coordinate system you are using: first corresponds to the first x-y coordinate system, scr (short for screen) is for screen coordinates. You can learn more by looking in the documentation for drawing circles.