# Re: [R] srt --- slope text with function?

From: ivo welch <ivowel_at_gmail.com>
Date: Sun 05 Feb 2006 - 10:02:22 EST

Some more experimentation reveals that I need to also adjust for the plot dimensions. (And there are other issues I probably do not know yet and totally misprogrammed...just trying to be helpful.)

native.slope <- function( x, y, where.i,

```			 xlim = par()\$xaxp, ylim= par()\$yaxp,
asp.ratio = (par()\$fin)[1]/(par()\$fin)[2],
debug =0) {
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");

## alas, we also need to take into account the plot region:   pq= par()\$plt; plt.distort= (pq[2]-pq[1])/(pq[4]-pq[3]);

net.slope= d/asp.ratio/d.m / plt.distort;

slope = atan(net.slope)/pi*180.0;

if (debug) {
cat("xlim=", par()\$xaxp, "\n");
cat("ylim=", par()\$yaxp, "\n\n");

```    cat("native.slope: d=", d, " (",y[where.i-1],y[where.i+1],
x[where.i-1], x[where.i+1],")",
"d.m=",d.m, " (", ylim[2],ylim[1],xlim[2],xlim[1], ")",
"asp.ratio=", (par()\$fin)[1], ":", (par()\$fin)[2], "==>", net.slope,
```
"=", slope, "deg\n");
}

return( slope = slope );
}

On 2/4/06, ivo welch <ivowel@gmail.com> wrote:
> 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
>
> ################################################################
> #### 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.)
> ################################################################
> [old function deleted]
>
> # 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 <murdoch@stats.uwo.ca> 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
> >
>

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