Thank you, Duncan. This led me to the info I needed. Here is a simple utility function that does what I needed---maybe it will come in helpful for others.
################################################################ #### native.slope computes a suitable srt from a function around #### a point on a function. This is useful until text() gets #### an srt parameter that is relative to the coordinate system. #### (Ideally, R would be able to slope along a function.) ################################################################ native.slope <- function( x, y, where.i, xlim = par()$xaxp, ylim= par()$yaxp, asp.ratio = (par()$fin)[1]/(par()$fin)[2] ) { if (where.i<=1) { return(0); } if (where.i>=length(y)) { return(0); } if (length(x)!=length(y)) { stop("native.slope: Sorry, but x and y must have equal dimensions, not ", length(x), " and ", length(y), "\n"); } # native slope in a 1:1 coordinate system d= ( (y[where.i-1]-y[where.i+1])/(x[where.i-1]-x[where.i+1]) ); if (is.na(d)) return(0); # we do not know how to handle an undefined spot at a function! d.m= (ylim[2]-ylim[1])/(xlim[2]-xlim[1]); # now adjust by the axis scale if (is.na(d)) stop("native.slope: internal error, I do not have sensible axis dimensions (", xlim, ylim, ")\n"); if (is.na(asp.ratio)) stop("native.slope: internal error, I do not have a reasonable drawing aspect ratio"); net.slope= d/asp.ratio/d.m; return(slope = atan(net.slope)/pi*180.0 ) } # some test code x<- seq(-10,20,by=0.1) y<- x*x; plot( x, y, type="l" ); display= ((1:length(y))%%40 == 0) for (i in 1:(length(y))) { if (display[i]) { points(x[i],y[i], pch=19); srt= native.slope( x, y, i ); text( x[i], y[i], paste(i,"=",x[i],"=",srt), srt=srt, cex=0.9 ); } } On 2/4/06, Duncan Murdoch <[EMAIL PROTECTED]> wrote: > On 2/4/2006 3:50 PM, ivo welch wrote: > > [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? > > I haven't done this, but you can presumably work it out from the > conversions implied by the "fig", "fin", "plt", and/or "usr" values. > > > > (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. > > Using strsplit and strwidth you should be able to do it, but it will > probably look quite ugly. > > Duncan Murdoch > ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html