[R] How to adjust the start of a series to zero? (i.e. subtract the first value from the sequence)

[Kristiina Hurme]

Hello, 
I'd have a time series, where I am plotting the means and sd of a distance
for a variety of positions along a bird's bill. I'd like to set each line
(represented by "point") to start at zero, so that I can look at the
absolute change along the series. At the moment I only know how to do that
in Excel, by subtracting the value of time 1, point 1 from all other times
for point 1. My actual data set has many points ( 20 per bird, only 3 shown
here), so I would love to make this faster in R. Ideally, I would have
another column titled "adj_mean" for the adjusted means. 

Here is an example. 

> sort2v4
    point time      mean        sd
1       1    1 52.501000 1.5073927
3       1    2 54.501818 0.8510329
4       1    3 56.601739 1.5787222
5       1    4 57.200000 1.2292726
6       1    5 59.300000 2.2632327
7       1    6 57.800893 1.4745218
8       1    7 55.303508 2.2661855
9       1    8 51.100943 1.8540025
10      1    9 50.600000 1.7126977
2       1   10 52.904716 1.1010460
111     2    1 50.605963 1.2633969
113     2    2 52.203828 0.7890765
114     2    3 54.100909 1.1013344
115     2    4 55.000000 1.1547005
116     2    5 57.001725 1.6341500
117     2    6 55.003591 1.5652438
118     2    7 52.911089 1.7373914
119     2    8 49.204022 1.0350809
120     2    9 48.904103 0.8747568
112     2   10 50.915700 0.8765483
131     3    1 48.608228 0.8433913
133     3    2 49.307101 0.4827703
134     3    3 51.310824 0.9424023
135     3    4 52.413350 0.6997860
136     3    5 54.116723 1.1927297
137     3    6 52.618161 1.1686288
138     3    7 49.822764 1.6303473
139     3    8 47.107336 1.2013356
140     3    9 47.104214 1.1986148
132     3   10 48.719484 0.6765047

and I would like it to look like this... (which I did in Excel). The start
of each time 1-10 has an adj_mean of 0. 
> sort2v4					
	point	time	mean	sd	adj_mean
1	1	1	52.501	1.5073927	0
3	1	2	54.501818	0.8510329	2.000818
4	1	3	56.601739	1.5787222	4.100739
5	1	4	57.2	1.2292726	4.699
6	1	5	59.3	2.2632327	6.799
7	1	6	57.800893	1.4745218	5.299893
8	1	7	55.303508	2.2661855	2.802508
9	1	8	51.100943	1.8540025	-1.400057
10	1	9	50.6	1.7126977	-1.901
2	1	10	52.904716	1.101046	0.403716
111	2	1	50.605963	1.2633969	0
113	2	2	52.203828	0.7890765	1.597865
114	2	3	54.100909	1.1013344	3.494946
115	2	4	55	1.1547005	4.394037
116	2	5	57.001725	1.63415	6.395762
117	2	6	55.003591	1.5652438	4.397628
118	2	7	52.911089	1.7373914	2.305126
119	2	8	49.204022	1.0350809	-1.401941
120	2	9	48.904103	0.8747568	-1.70186
112	2	10	50.9157	0.8765483	0.309737
131	3	1	48.608228	0.8433913	0
133	3	2	49.307101	0.4827703	0.698873
134	3	3	51.310824	0.9424023	2.702596
135	3	4	52.41335	0.699786	3.805122
136	3	5	54.116723	1.1927297	5.508495
137	3	6	52.618161	1.1686288	4.009933
138	3	7	49.822764	1.6303473	1.214536
139	3	8	47.107336	1.2013356	-1.500892
140	3	9	47.104214	1.1986148	-1.504014
132	3	10	48.719484	0.6765047	0.111256

Thank you so much for your help. 
Kristiina

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