how to move axes to center of chart?

Question

I need to plot a function on a coordinate system that has its origin at the center of the screen (or somewhere near the center, but not necessarily in the center) and I need to draw axes so that they cross at the origin. Axes should have labels and tics as well, and arrows.

I have no idea how to do this efficiently, so far in my code I manually set offsets for my tics, and draw arrows manually using offsets as well. I also set offsets on the axes labels. All this is very fragile, and offsets change depending on the terminal settings.

Would someone please help me with a sample code or an explanation how to do this the right way?

Answer 1

Well, using _zeroaxis is the "right" way to go to get the tics and labels in: setting the _range gives you the symmetry to make 0,0 centre ... and once you know the _range, you can draw arrows on manually.

set xzeroaxis
set xtics axis
set xrange [-10:10]
set arrow 1 from -9,0 to -10,0
set arrow 2 from  9,0 to  10,0

set yzeroaxis
set ytics axis
set yrange [-1:1]
set arrow 3 from 0,-.9 to 0,-1
set arrow 4 from 0,.9  to 0,1

set border 0

plot sin(x)

Answer 2

Hack attack!

set term pngcairo truecolor size 300,300 font "Arial,12"
set out 'plot.png'

# x,y min/max and center
xmin = -10 
xc = 0 
xmax = 10
ymin = -2
yc = 0 
ymax = 2 
# default borders
tm = 1 
bm = 1 
rm = 4 
lm = 4 
# arrow scale factor to cover last tic 
af = 1.05
set arrow from xc,yc to xmin*af,yc filled size 0.6,30
set arrow from xc,yc to xmax*af,yc filled size 0.6,30
set arrow from xc,yc to xc,ymax*af filled size 0.6,30
set arrow from xc,yc to xc,ymin*af filled size 0.6,30

set multiplot layout 2,2 
## Plot 1, top left
set key top left
set xr [xmin:xc]
set yr [yc:ymax]
set tmargin tm
set bmargin 0
set rmargin 0
set lmargin lm
set border 9
unset ytics
set xtics nomirror
plot sin(x)
## Plot 2, top right
unset key
set xr [xc:xmax]
set lmargin 0
set rmargin rm
set border 3
set ytics nomirror
replot
## Plot 3, bottom left
set xr [xmin:xc]
set yr [ymin:yc]
set bmargin bm
set tmargin 0
set lmargin lm
set rmargin 0
set border 12
unset tics
replot
## Plot 4, bottom right
set xr [xc:xmax]
set lmargin 0
set rmargin rm
set border 6
set ytics nomirror
replot
unset multiplot

I get this output:

That said, you might look into the zeroaxis option. There is a demonstration here. Sadly this option doesn't draw the axis at zero, but just puts a line at that position.

I would not say my method is 'efficient,' but there is probably not too much effort to modify this for your purposes apart from changing the function/data being plotted since most of the work is done by replot commands. I don't know an option to make gnuplot put arrows at the end of axes or change where the axes are drawn natively.

Answer 3

In addition to Jim's answer, if required the tics and axes can be brought to the front

# bring the grid over the plot
set grid front

# remove grid if not required
unset grid