[R] Obtain gradient at multiple values for exponetial decay model

[g l]

Readers,

Data set:

t,c
0,100
40,78
80,59
120,38
160,25
200,21
240,16
280,12
320,10
360,9
400,7

graphdata<-read.csv('~/tmp/data.csv')
graphmodeld<-lm(log(graphdata[,2])~graphdata[,1])
graphmodelp<-exp(predict(graphmodeld))
plot(graphdata[,2]~graphdata[,1])
lines(graphdata[,1],graphmodelp)

Please what is the function and syntax to obtain gradient values for the model curve at various requested values, e.g.:

when graphdata[,1] at values = 100, 250, 350 ?