[R] plot different groups as factors

[Luigi Marongiu]

dear all,
i would like to plot the value of different response groups. when i simply
use  plot(y ~ x) i obtain a series of boxplots. i would rather use dots. i
also tried with stripchart(y ~ x), which gives better results but does not
place properly the labels since place them alphabetically.
in addition i actually have 6 response groups: 3 classes (low, medium,
high) and 2 sampling time (12 and 18 months).
how can i generate these individual groups and plot them in the correct
order (low, medium, high and 12, 18)? i believe is something to do with the
factors but i don't know how to implement them.
what i am looking for is to generate a figure such as the one i sketched in
the attached file. i also attached a dataframe version of my data. the
vectors containing the same data are:
time <-c(  18,    18,    18,    18,    18,    18,    18,    18,    18,
18,    18,    18,    18,    18,    18,    18,    18,    18,    18,
18,    18,    18,    18,    18,    18,    18,    18,    18,    12,
12,    12,    12,    12,    12,    12,    12,    12,    12,    12,
12,    12,    12,    12,    12,    12,    12,    12,    12,    12,
12,    12,    12,    12,    12,    12,    12,    12,    12,    12,
12,    12,    12,    12,    12,    12,    12)
class <-c(    medium,    medium,    medium,    medium,    medium,
medium,    medium,    medium,    medium,    medium,    medium,
medium,    medium,    medium,    high,    high,    high,    high,
high,    high,    high,    low,    low,    low,    low,    low,    low,
low,    medium,    medium,    medium,    medium,    medium,    medium,
medium,    medium,    medium,    medium,    medium,    medium,
medium,    medium,    medium,    medium,    medium,    medium,    high,
high,    high,    high,    high,    high,    high,    high,    high,
high,    low,    low,    low,    low,    low,    low,    low,    low,
low,    low)
value <-c(    2.92,    0.01,    0.36,    3.16,    0.99,    0.38,
0.01,    5.1,    0.04,    0.01,    1.33,    4.13,    0.15,    0.15,
14.18,    4290.14,    26.8,    5.33,    17.58,    14.29,    248.5,
0.01,    0,    0,    0,    0,    0,    0,    0.151395382,
5.327863403,    5.10096383,    1.32567787,    4.352404124,
0.458606982,    2.915908912,    0.011996374,    0.364710382,
0.033016026,    3.161701212,    0.381564497,    0.010971385,
0.035646472,    0.014781805,    4.129708296,    0.153094117,
0.018497847,    15.09178491,    17.58393041,    14.17643928,
4290.143561,    26.79730719,    294.6367065,    14.2888441,
248.495231,    209.3131795,    2014.506722,    0.010751273,
0.002325138,    0.000637473,    0.003984336,    0.006018154,
0.003620907,    0.0000745936,    0.000142311,    0.002460417,
0.001280189)

Thank you very much for any help you could provide!
regards
Luigi
-------------- next part --------------
	time	class	value
1	18	medium	2.92
2	18	medium	0.01
3	18	medium	0.36
4	18	medium	3.16
5	18	medium	0.99
6	18	medium	0.38
7	18	medium	0.01
8	18	medium	5.1
9	18	medium	0.04
10	18	medium	0.01
11	18	medium	1.33
12	18	medium	4.13
13	18	medium	0.15
14	18	medium	0.15
15	18	high	14.18
16	18	high	"4,290.14"
17	18	high	26.8
18	18	high	5.33
19	18	high	17.58
20	18	high	14.29
21	18	high	248.5
22	18	low	0.01
23	18	low	0
24	18	low	0
25	18	low	0
26	18	low	0
27	18	low	0
28	18	low	0
29	12	medium	0.151395382
30	12	medium	5.327863403
31	12	medium	5.10096383
32	12	medium	1.32567787
33	12	medium	4.352404124
34	12	medium	0.458606982
35	12	medium	2.915908912
36	12	medium	0.011996374
37	12	medium	0.364710382
38	12	medium	0.033016026
39	12	medium	3.161701212
40	12	medium	0.381564497
41	12	medium	0.010971385
42	12	medium	0.035646472
43	12	medium	0.014781805
44	12	medium	4.129708296
45	12	medium	0.153094117
46	12	medium	0.018497847
47	12	high	15.09178491
48	12	high	17.58393041
49	12	high	14.17643928
50	12	high	4290.143561
51	12	high	26.79730719
52	12	high	294.6367065
53	12	high	14.2888441
54	12	high	248.495231
55	12	high	209.3131795
56	12	high	2014.506722
57	12	low	0.010751273
58	12	low	0.002325138
59	12	low	0.000637473
60	12	low	0.003984336
61	12	low	0.006018154
62	12	low	0.003620907
63	12	low	7.46E-05
64	12	low	0.000142311
65	12	low	0.002460417
66	12	low	0.001280189