R help - Oct 2009 - plot scale

Hi,

Is there a way to set the scale of a plot (i.e. number of axis units
per centimeter) when you output it to postscript? If not, how am I
supposed to plot graphs with different axis limits to the same scale?
They just get resized to fit the paper so that graphs which show a
smaller number of axis units end up with a larger scale.

Cheers,

Ben

-- 
Dr. Ben Kenward
Department of Psychology, Uppsala University, Sweden
http://www.benkenward.com

On 02/10/2009 4:07 AM, Ben Kenward wrote:
> Hi,
> 
> Is there a way to set the scale of a plot (i.e. number of axis units
> per centimeter) when you output it to postscript? If not, how am I
> supposed to plot graphs with different axis limits to the same scale?
> They just get resized to fit the paper so that graphs which show a
> smaller number of axis units end up with a larger scale.
I don't think there's a simple way to specify the exact relationship, 
but you can do it with some work.

When you open the postscript device, you can specify the size.  When you 
do a plot, you can specify the size of the margins, so that lets you 
indirectly specify the size of the plot region.  And of course you can 
specify the axis limits.

So the way I'd do what you want is as follows:  plot the data, use 
par("usr") to extract the axis limits, use par("mai") to
extract the
margin size.  Then calculate the size you want for the particular 
scaling, and open the postscript device with that size, reset
par("mai")
to the previous value, and repeat your plot.

Duncan Murdoch

> 
> Hi,
> 
> Is there a way to set the scale of a plot (i.e. number of axis units
> per centimeter) when you output it to postscript? If not, how am I
> supposed to plot graphs with different axis limits to the same scale?
> They just get resized to fit the paper so that graphs which show a
> smaller number of axis units end up with a larger scale.
> 
> Cheers,
> 
> Ben
> 
If I understand you correctly, you want the units on each axis to 
be physically represented equally on the same scale.  If this is right, 
try using the lattice package (xyplot, etc.) with aspect="iso".

Lattice lets you adjust the aspect ratio of the plot to whatever you 
like and constrains the dimensions of the plot accordingly.  Base 
graphics also allow for control of the aspect ratio using "asp=", but 
extend the limits of the plot to fill whatever region you are plotting 
in.

Here is a different approach:
This example uses the default plotting device, but should work the same with
postscript or any other device.  Just set the size of the paper to a standard
size, or a large enough size to fit your largest plot:

dev.new()
tmp <- par('plt')

scale <- 5
x <- runif(100, 0, 20)
y <- rnorm(100, x)

dx <- diff(range(x))*1.08
wx <- grconvertX(dx/scale, from='inches', to='ndc')

dy <- diff(range(y))*1.08
wy <- grconvertY(dy/scale, from='inches', to='ndc')

par(plt = c(tmp[1], tmp[1]+wx, tmp[3], tmp[3]+wy) )
plot(x,y)

dev.new()

x2 <- runif(100, 0,5)
y2 <- rnorm(100, x2)

dx <- diff(range(x2))*1.08
wx <- grconvertX(dx/scale, from='inches', to='ndc')

dy <- diff(range(y2))*1.08
wy <- grconvertY(dy/scale, from='inches', to='ndc')

par(plt = c(tmp[1], tmp[1]+wx, tmp[3], tmp[3]+wy) )
plot(x2,y2)


The above only works for 1 plot per page (no layout or par(mfrow=... ) ).

This keeps the lower left corner of the plot in the same place (so change the
margins before the call to par('plt') if you want them different). Then
sets the top and right parts of the plotting region leaving white space on the
top and right to get the scaling correct (you will have problems if your desired
scale does not fit in the window/on the page).  This example uses a
"scale" of 5 plot units per inch, change as appropriate.  The *1.08 is
to account for the extra area that is added to the plot by default, change this
for the other axis type.

Hope this helps,



-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Ben Kenward
> Sent: Friday, October 02, 2009 2:08 AM
> To: r-help at r-project.org
> Subject: [R] plot scale
> 
> Hi,
> 
> Is there a way to set the scale of a plot (i.e. number of axis units
> per centimeter) when you output it to postscript? If not, how am I
> supposed to plot graphs with different axis limits to the same scale?
> They just get resized to fit the paper so that graphs which show a
> smaller number of axis units end up with a larger scale.
> 
> Cheers,
> 
> Ben
> 
> --
> Dr. Ben Kenward
> Department of Psychology, Uppsala University, Sweden
> http://www.benkenward.com
> 
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