[R] slope estimations of teeth like data

[Petr Pikal]

Dear all

Suppose I have teeth like data similar like

x <- 1:200
y <- 0.03*x[1:100]+rnorm(100, mean=.001, sd=.03)
z <- 3-rep(seq(1,100,10),each=10)*.03+rnorm(100,mean=.001, sd=.03)
plot(x,c(y,z))

and I want to have a gradient estimations for some values from increasing part of 
data

like

y.agg <- aggregate(diff(c(y,z)), list(rep(seq(1,200,10),each=10)[1:199]), mean)

y.agg[1:10,]  ##is OK, I want that
y.agg[11:20,] ##is not OK, I do not want that

actual data are similar but more irregular and have subsequent gradual increases 
and decreases, more like

set.seed(1)
yy<-NULL
for( i in 1:10) yy <- c(yy,c(y,z)[1:floor(runif(1)*200)])
length(yy)
[1] 1098

plot(1:1098,yy)

Is there anybody who has some experience with such data, mainly how to extract 
only increasing portions or to filter values of "yy" such as only aggregated slopes 
from increasing parts are computed and other parts are set to NA. Sometimes 
actual data have so long parts of steady or even slightly increasing values at 
decreasing part that aggregated values are slightly positive although they are 
actually from decreasing portion of data.

Thank you
Petr Pikal
petr.pikal at precheza.cz