How to set the length of ticks to be exactly 1 inch?

Question

 x<-seq(-3,3,by=0.1)
 y=0.5*x^3 
 ## Set png size to be slightly bigger than plot to take account of one line margin
 png( "~/myplot.png" , width = 600 , height = 2600 ,units="px",res=100)
 ## Set graphical parameters to control the size of the plot area and the margins
 ## Set size of plot area to 6 inches wide by 15 inches tall
 par( pin = c(6,26) )
 ## Remove margins around plot area
 par( mai = c(0,0,0,0) )
 ## Remove outer margins around plot area
 par( omi = c(0,0,0,0) )
 ## Set y-axis take use entire width of plot area (6 inches) and to have 7 tick marks (-3,-2,-1,0,1,2,3)
 par( xaxp = c(-3,3,7) )
 ## Set x-axis take use entire height of plot area (26 inches) and to have 27 tick marks (-13,-12,...,0,...11,12,13)
 par( yaxp = c(-13,13,27) )
 ## Create the plot
 plot( x , y , type = "l" , frame.plot = FALSE , axes = FALSE )
 axis( 1 , pos = 0 , at = seq( -3 , 3 , by = 1 ) , labels = seq( -3 , 3 , by = 1 ) )
 axis( 2 , pos = 0 , at = seq( -13 , 13 , by = 1 ) , labels = seq( -13 , 13 , by = 1 ) )
 text(0.5,5,expression(f(x)==frac(1,2)*x^3) )
 ## Turn PNG device off
 dev.off()

We can get the graph which is 6 inches width, 26 inches height, but the tick unit in the x-axis or y-axis is not 1 inch; please see the attachment g1.png:

The reason is R will not use the entire space to set the x-axis and y-axis, R keep little space; please see the attachment g2.png:

I can png( "~/myplot.png" , width = 610 , height = 2700 ,units="px",res=100), but how can I make the physical length of tick unit just to be 1 inch?

Answer 1

To specify the exact settings you want it is best to output to PDF. The onscreen plot can change when it is re-sized etc. You need to set various graphical parameters to take care of the margins. There is a good outline of the plotting area in base graphics and the appropriate par names to adjust here. This code should give you exactly what you asked for:

Edit

It looks like the PDF device keeps a border of one line around the outside, no matter what. Be default line height ~ 0.2 inches. Therefore we increase our width and height by 0.5 inches and we get spacing of exactly one inch.

Here is the measurements on my computer!!

## Set pdf size to be slightly bigger than plot to take account of one line margin
pdf( "~/myplot.pdf" , width = 6.5 , height = 15.5 )

## Set graphical parameters to control the size of the plot area and the margins
## Set size of plot area to 6 inches wide by 15 inches tall
par( pin = c(6,15) )

## Remove margins around plot area
par( mai = c(0,0,0,0) )

## Remove outer margins around plot area
par( omi = c(0,0,0,0) )

## Set y-axis take use entire width of plot area (6 inches) and to have 7 tick marks (-3,-2,-1,0,1,2,3)
par( yaxp = c(0,1,7) )

## Set x-axis take use entire height of plot area (15 inches) and to have 27 tick marks (-13,-12,...,0,...11,12,13)
par( xaxp = c(0,1,27) )

## Create the plot
plot( x , y , type = "l" , frame.plot = FALSE , axes = FALSE )
axis( 1 , pos = 0 , at = seq( -3 , 3 , by = 1 ) , labels = seq( -3 , 3 , by = 1 ) )
axis( 2 , pos = 0 , at = seq( -13 , 13 , by = 1 ) , labels = seq( -13 , 13 , by = 1 ) )
text(0.5,5,expression(f(x)==frac(1,2)*x^3) )

## Turn PDF device off
dev.off()

And the results: