[R] srt --- slope text with function?

[ivo welch]

[resent, plus small addition; I do not understand why gmail sent a
weird charset.]

Dear R wizards:

I would love to write a general function that matches the slope of a plotted
line in an xy-plot at a particular x,y location.  something like

   x<- (1:10)^2; y<- 40:50;
   plot( x,y, type="l", xlim=c(0,90) )
   srt.at5 = text.at.current.plot.with.slope( x, y,  5);
   text( x[5],y[5], pos=3, srt=srt.at.5);

to do this, I first need to compute the function slope around x[5], which is
an easy task.  alas, the harder task is that I need to scale this by the
plot aspect ratio and the axes.  How can a function read this from the
current plot?

(Has someone written such a function, perhaps more embellished, to save me
the debugging effort?)

Or, is there an alternative to srt, which slopes the text relative to the
existing scale?

  *** come to think of it, what I would really like is the ability of
text to 'snake' itself along the line itself.  I doubt that this is
easily possible, but I just wanted to ask.

help appreciated.

sincerely,

/ivo welch