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

[Duncan Murdoch]

I don't think it's got the slope exactly right - you'll see this if you 
go to a really extreme aspect ratio by changing the shape of a window to 
be long and thin before you call your code.  To fix this:

  - use "usr" rather than "xaxp" and "yaxp" to get the limits of the 
plot region in user coordinates; those two refer to the ticks, not the 
whole plot region as "usr" does.

- "fin" is the whole figure region, not just the plot region; you need 
to use "plt" to modify it to find the plot region within it.  So I think 
the aspect ratio should really be done as

   pars <- par("fin", "plt")

   asp.ratio <- (diff(pars$plt)[1]*pars$fin[1]) /
                (diff(pars$plt)[3]*pars$fin[2])

Some other suggestions:

  - split the function into two:  one that determines a slope from the 
data, and one that converts a slope to an angle suitable for "srt".  (I 
think the latter would have pretty wide use; the former is pretty 
specialized for data the way you're using it).

  - use the fact that defaults in a function call can be local variables 
in the function, so that you only need one call to par() instead of 4. 
(The 4 calls probably take a negigible amount of time, but it just looks 
wasteful to make them.)

Duncan Murdoch

On 2/4/2006 5:19 PM, ivo welch 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
> 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 <murdoch at 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
>>