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

[g l]

> Sent: Thursday, April 05, 2018 at 4:40 PM
> From: "Jeff Newmiller" <jdnewmil using dcn.davis.ca.us>
> 
> the coef function.
> 

For the benefit of other novices, used the following command to read the documentation:

?coef

Then tried and obtained:

> cvalue100<-coef(graphmodelp~100)
> cvalue100
NULL

Then looked at the model values which of course correspond to original non-modelled values.

graphmodelp
        1         2         3         4         5         6         7         8 
91.244636 69.457794 52.873083 40.248368 30.638107 23.322525 17.753714 13.514590 
        9        10        11 
10.287658  7.831233  5.961339

This prompted to think that interpolation is required, but the function 'approx' only seems to perform constant interpolation.

Is the correct thinking to find a function to perform interpolation, then find/write a function to differentiate the model at a specific value of x, to find gradient at that point?