[R] how to contour plot on Date data.

[Mao Jianfeng]

Dear R-listers,

I have a data set which combine population (site name), longitude,
latitude, altitude, initial (for flowering of a plant), and end (also
for flowering of the same plant) of each site.

Now, I want to make contour plot to display spatial pattern of the
initial or end of the
 flowering especially, I still can not plot that, although I have
gotten some helps from this list. I need to know how can I process my
data so as to meet the ascending order of X and Y. And, I need to know
if if contour can handle Date variable.

Please give me some directions on that. Thank you in advance.

data and codes used are as followed:

1. data
Population      Lon     Lat     Alt     Initial End
ZDPd01  98.8763 28.4419 3449    2009-4-15       2009-4-25
GSPy01  98.63446667     28.01211667     1736    2009-3-15       2009-3-28
KDPd01  101.9054667     30.18946667     2914    2009-4-18       2009-4-29
MKPd01  102.2027833     31.90801667     2526    2009-4-22       2009-5-1
LXPd01  102.8014833     31.66243333     2746    2009-4-9        2009-4-21
LXPd02  103.0387        31.40156667     2158    2009-4-7        2009-4-19
BXPd01  102.75395       30.71616667     2180    2009-4-5        2009-4-14
XJPd01  102.5218        31.02406667     2467    2009-4-2        2009-4-9
BJPt01  115.95  40.45   80      2009-4-19       2009-4-26
KMPd01  102.6225761     24.96880965     1900    2009-3-22       2009-4-11
NYPd06  94.08488239     29.73718216     3379.591549     2009-5-1        2009-5-7
PLPd02  95.49816933     29.90361244     2650.12 2009-4-26       2009-5-5
CYPd01  97.01336034     28.50232574     1631.607595     2009-4-15
 2009-4-23
CYPd02  97.47131095     28.67033436     2431.802469     2009-4-21       2009-5-2
CYPd03  97.22515032     29.13309676     3263.019417     2009-4-25       2009-5-3
NCPt01  118.97  42.28   1300    2009-5-25       2009-6-1
QLPd01  103.6905774     37.44786532     2403.370968     2009-5-15
 2009-5-23
LKSPt01 112.0359546     36.61293704     1664.597938     2009-4-21
 2009-5-11

2. code

phen<-read.delim("2009phenology.txt", header=T)
str(phen)
phen.1<-phen[,1:7]
phen.1[,6]<-as.Date(phen.1[,6])
phen.1[,7]<-as.Date(phen.1[,7])
str(phen.1)

phen.2<-phen.1[order(phen.1[,3], phen.1[,4]),]
fix(phen.2)

contour(phen.2$Lon, phen.2$Lat, phen.2$Initial) # failed
contour(phen.2$Lon, phen.2$Lat, phen.2$Alt) # failed

Regards,

Mao J-F