[R] mean on a plot

[romzero]

Hi, i need to draw a plot with means of values, reading a table from
datafile.

Example of datafile:

Days	Weight
0	178.00
0	250.00
0	242.00
0	239.00
0	223.00
0	188.00
0	237.00
0	212.00
0	273.00
0	191.00
0	173.00
0	233.00
0	227.00
0	253.00
0	232.00
4	177.00
4	249.00
4	241.00
4	238.00
4	222.00
4	188.00
4	236.00
4	211.00
4	272.00
4	190.00
4	172.00
4	232.00
4	226.00
4	252.00
4	231.00

if i use "plot(Weight~Days, data=la)" the result is this
http://r.789695.n4.nabble.com/file/n3031068/weightm.jpg 
 
how can i draw a plot with means for each days?

tnx for help :)

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