R help - Mar 2010 - problem with abline and lines

Dear R users,

I need to plot to perpendicular straight lines. However, although I set the
coefficients
so that the lines are perpendicular, they do not look to be so in the plot. Here
is a minimal working example:
plot(x=c(-1, 1), y=c(-1, 1)); abline(a=0, b=1/sqrt(2)); abline(a=0,
b=-1/sqrt(2))

Please tell me if the same problem is valid by you. I am running R-2.10.1 on
Linux.

Is there a way out of this?

Regards,

Martin

Hi!

You forgot to invert the slope: a perpendicular of 1/sqrt(2) should be
-sqrt(2).
Also, you should add asp=1 in the plot command to lock the aspect ratio,
otherwise the scale of both X and Y may be different according to the size
of the window:
plot(x=c(-1, 1), y=c(-1, 1),asp=1); abline(a=0, b=1/sqrt(2)); abline(a=0,
b=-sqrt(2))

Miguel


On Mon, Mar 22, 2010 at 11:13 AM, Martin Ivanov <tramni@abv.bg> wrote:
>  Dear R users,
>
> I need to plot to perpendicular straight lines. However, although I set the
> coefficients
> so that the lines are perpendicular, they do not look to be so in the plot.
> Here is a minimal working example:
> plot(x=c(-1, 1), y=c(-1, 1)); abline(a=0, b=1/sqrt(2)); abline(a=0,
> b=-1/sqrt(2))
>
> Please tell me if the same problem is valid by you. I am running R-2.10.1
> on Linux.
>
> Is there a way out of this?
>
> Regards,
>
> Martin
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
	[[alternative HTML version deleted]]

On 22-Mar-10 11:13:39, Martin Ivanov wrote:
>  Dear R users,
> I need to plot to perpendicular straight lines. However, although
> I set the coefficients so that the lines are perpendicular, they
> do not look to be so in the plot. Here is a minimal working example:
> 
> plot(x=c(-1, 1), y=c(-1, 1)); abline(a=0, b=1/sqrt(2)); abline(a=0,
> b=-1/sqrt(2))
> 
> Please tell me if the same problem is valid by you. I am running
> R-2.10.1 on Linux.
> 
> Is there a way out of this?
> 
> Regards,
> Martin
Hi Martin,
For one thing, your coefficients are wrong for orthogonality.
The condition for y = a1 + b1*x and y = a2 + b2*x to be perpendicular
to each other is that b1*b2 = -1. It will look much better with

  plot(x=c(-1, 1), y=c(-1, 1)); 
  abline(a=0, b=1/sqrt(2)); 
  abline(a=0, b=-sqrt(2))

but probably will not look quite right, because the plot() command
does not necessarily ensure that the x and y axes are identically
scaled (in screen coordinates) when they appear on screen.

To ensure this as well, you need to set the aspect-ratio to be 1:

  plot(x=c(-1, 1), y=c(-1, 1), asp=1);
  abline(a=0, b=1/sqrt(2)); 
  abline(a=0, b=-sqrt(2))

It can take a little while to discover the parameter
"asp".
>From ?plot (which does not mention it) you will find a suggestion
to look at ?plot.default for further details, where you can read:

  plot(x, y = NULL, type = "p",  xlim = NULL, ylim = NULL,
       log = "", main = NULL, sub = NULL, xlab = NULL, ylab = NULL,
       ann = par("ann"), axes = TRUE, frame.plot = axes,
       panel.first = NULL, panel.last = NULL, asp = NA, ...)

and:

  asp: the y/x aspect ratio, see 'plot.window'. And then ?plot.window

tells you all about it:

  asp: If 'asp' is a finite positive value then the window is set up
       so that one data unit in the x direction is equal in length
       to 'asp' * one data unit in the y direction.

This even applies if you distort a previous plot window by dragging
one edge of it. For example, kill any existing plot window, and then
execute the above to create a new one. Then, say, drag the left-hand
edge so as to halve the width without changing the height. Then
repeat the plot commands. You will find that in the new plot the
two lines are again perpendicular, the shape of the window has
not changed, but the y-axis has now been extended numerically so that
the aspect ratio (as defined above) is still 1.0!

Hoping this helps!
Ted.

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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 22-Mar-10                                       Time: 11:43:32
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