Canvas size in gnuplot

Current status

At the moment, the definition of the plot size in gnuplot is somehow unsatisfactory. Depending on the terminal, set size reacts differently. For example, in the png terminal, the position of the size command does not make a difference while in the postscript terminal, set size before set terminal is different from after it. In addition, set size can change the canvas of the output even if the output size is defined explicitely.

For example,

set size 1,2
set terminal png size 320,240

leads to an output file with 320 × 480 pixels.

Proposal for a new structure

• The canvas size is defined by the set terminal command exclusively.
• By default, the lower left corner of the canvas has the screen coordinates 0,0 and the upper right corner has the screen coordinates 1,1. (This is already the case for terminals that have an option set terminal size.)
• set size only changes the size of the plot, not the canvas size. At the moment, this is already the case for multiplots.
• A new command is introduced that changes the screen coordinates of the upper right corner (and optionally of the lower left corner) without changing the canvas. This command could be named set screen-coordinate (which is a bad name, we should look for a better one). The screen-coordinate command is useful to select a special aspect ratio of the screen coordinate system.

All plots use the include file canvas.inc. In addition, all plots use the patch bbsize-20051012-1450.diff to enable labels outside the range of screen coordinates 0,0 to 1,1.

Default case

The default is a plot with screen coordinates 0,0 and 1,1 in the lower left and upper right corner, respectively:

Produced by:

set size 1,1
set terminal png size 320,240
set output 'canvas1.png'
load 'canvas.inc'
plot sin(x) notitle
set output

set screen-coordinate min 0,0 max 1,1
set terminal png size 320,240
set output 'canvas1.png'
load 'canvas.inc'
plot sin(x) notitle
set output

In all plots, the green arrows denote a screen coordinate range of width 0.4 and height 0.4.

Double height of the plot with max screen y coordinate of 1

Screen coordinates 0,0 and 1,1 in the lower left and upper right corner, respectively:

Produced by:

set size 1,1
set terminal png size 320,480
set output 'canvas4.png'
load 'canvas.inc'
plot sin(x) notitle
set output

set screen-coordinate min 0,0 max 1,1
set terminal png size 320,480
set output 'canvas4.png'
load 'canvas.inc'
plot sin(x) notitle
set output

Here, a plot with the size 1,1 in screen coordinates has a different size and aspect ratio than in the default case. Also, the green coordinate arrows have a different aspect ratio.

In many cases, this behaviour is intended by the user.

Double height of the plot with max screen y coordinate of 2

Screen coordinates 0,0 and 1,2 in the lower left and upper right corner, respectively:

Produced by:

set size 1,2
set terminal png size 320,240
set output 'canvas2.png'
load 'canvas.inc'
plot sin(x) notitle
set output

set screen-coordinate min 0,0 max 1,2
set terminal png size 320,480
set output 'canvas4.png'
load 'canvas.inc'
plot sin(x) notitle
set output

The plot itself looks identical to the section before. But everything that is placed with explicit screen coordinates is placed differently. For example, the green coordinate arrows have the same size and aspect ratio as in the default case.

In many cases, also this behaviour is intended by the user.

The current syntax is unlogical here because set terminal png size 320,240 leads to a picture of the size 320 × 480. The proposed new syntax would me much more logical.

Multiplots (screen height 1)

Screen coordinates 0,0 and 1,1 in the lower left and upper right corner, respectively:

Produced by:

set size 1,1
set terminal png size 320,480
set output 'canvas5.png'
load 'canvas.inc'
set multiplot
set size 1,0.5
plot sin(x) notitle
set origin 0,0.5
replot
unset multiplot
set output

set screen-coordinate min 0,0 max 1,1
set terminal png size 320,480
set output 'canvas5.png'
load 'canvas.inc'
set multiplot
set size 1,0.5
plot sin(x) notitle
set origin 0,0.5
replot
unset multiplot
set output

In order to maintain two plots that have the same size as in the default case, their size has to be given as 1,0.5. Also, the green coordinate system scales by factor two in vertical direction.

Multiplots (screen height 2)

Screen coordinates 0,0 and 1,2 in the lower left and upper right corner, respectively:

Produced by:

set size 1,2
set terminal png size 320,240
set output 'canvas3.png'
load 'canvas.inc'
set multiplot
set size 1,1
plot sin(x) notitle
set origin 0,1
replot
unset multiplot
set output

set screen-coordinate min 0,0 max 1,2
set terminal png size 320,480
set output 'canvas3.png'
load 'canvas.inc'
set multiplot
set size 1,1
plot sin(x) notitle
set origin 0,1
replot
unset multiplot
set output

In order to maintain two plots that have the same size as in the default case, their size can still be given as 1,1. Also, the green coordinate system stays unchanged.

Conclusions

With current gnuplot, canvas sizes larger then 1,1 are not possible because the clipping algorithm inhibits that unnecessarily. With the patch bbsize-20051012-1450.diff, this problem can be solved. If the function of the set size command will not be changed in its basis, a patch similar to this one has to be applied in my opinion.

But there still is a problem because the set size command behaves in a strange manner because it can change the canvas size.

In my opinion, both scenarios are useful and necessary, namely maximal screen coordinates of 1,1 independent on the output size and the possibility to scale the screen axes in the bounds of the canvas, for example, to reach the same aspect ratio and scaling of the screen axes for different canvas sizes.

At last, there is a strong compatibility issue. In many of my documents, I have used eps and epslatex plots with a size smaller than 1,1 with extensive use of screen coordinates. Since neither postscript nor epslatex provide a size option, the screen coordinate system remained unchanged in all my plots, meaning that the upper right corner has the screen coordinate 0.7,0.8, for example. If a new syntax is introduced that supports this case, I just can change two lines in each plot instead of many. I believe that this problem applies to other users, too (as can be seen out of the bug message in the mailing list, if I remember correctly).