gnuplot: max and min values in a range - gnuplot

Question

I'm plotting some data with a different X range and I would like to change yrange according to the maximum and minimum value of the data in the current X range. When I use GPVAL_Y_MAX and GPVAL_Y_MIN, these values correspond to the maximum and minimum of the whole data, not just the data in the range.
For example, I have the following data:
1 3
2 5
3 8
4 20
5 30
I use the following script:
plot 'data.txt' u 1:2;
set xrange [1:3];
replot
set xrange [1:5];
replot
In the first plot I would like to set yrange in [3:8], but in the second plot the yrange sholud be [3:30]. If I use something like
set yrange [GPVAL_Y_MIN:GPVAL_Y_MAX]
GPVAL_Y_MIN and GPVAL_Y_MAX have the same value independently of the xrange.
Any solution?

Answer 1

The variables you want are GPVAL_DATA_Y_MIN and GPVAL_DATA_Y_MAX, which are the y-min/max of the data plotted in a certain range. GPVAL_Y_MIN and GPVAL_Y_MAX are a little less useful generally because they tell you where the edges of the plot border are (in general these values extend a little beyond the GPVAL_DATA... variables because gnuplot leaves a little space between the data and the edge of the plot).
To take advantage of these variables you have to use the range specifiers to the plot command:
plot [1:3] 'data.txt'
set yr [GPVAL_DATA_Y_MIN:GPVAL_DATA_Y_MAX]
replot
...
By the way, the u 1:2 specification is redundant unless you want to remind yourself of which columns you are plotting, since plotting the first two columns as x and y is the gnuplot default. If you don't want to replot to the same output terminal (which is not helpful in some terminals like eps where replotting makes a second page with the same plot), use this command sequence:
set terminal unknown
plot [1:3] 'data.txt'
set terminal <actual output terminal here>
set output 'output.trm'
plot [1:3][GPVAL_DATA_Y_MIN:GPVAL_DATA_Y_MAX] 'data.txt'
Note the use of the range specifier again, this time with a y range specified. This is a little more compact than specifying with set yrange, but makes for a longer line of code.
If you have gnuplot 4.6.0 or higher, you can take advantage of the stats command to avoid replotting. The stats command creates a bunch of handy variables
stats [1:3] 'data.txt'
plot [1:3][stats_min_y:stats_max_y] 'data.txt'
A slightly different command,
stats [1:3] 'data.txt'
plot [stats_min_x:stats_max_x][stats_min_y:stats_max_y] 'data.txt'
Would fill the plot in the x direction based on where the actual data lie. For instance if you had data points at {(1.1, 3), (2, 4), (2.9,5)}, the x range would be set to [1.1:2.9].

Answer 2

Setting the yrange to GPVAL_DATA_Y_MIN:GPVAL_DATA_Y_MAX has the disadvantage of not using gnuplots autoscaling functionality which extends the ranges to the next tic.
In automatic plotting I therefore prefer the following
f(x)=sin(x)>0.5? 1:-1 #example function
set ytics 0.2
plot 1.01*f(x) # dummy plot to set GPVAL_*
set yrange [GPVAL_Y_MIN:GPVAL_Y_MAX]
plot f(x) # actual plot
This also works for data plots of course:
plot 'data.csv' u 1:(1.01*$2)
set yrange [GPVAL_Y_MIN:GPVAL_Y_MAX]
plot 'data.csv' u 1:2

Answer 3

I use it like this to define an x range for a funcion
plot [0:5] sin(10*x) + cos(3*x)
Also, you can set the range before ploting
set xrange [0:5]
plot sin(10*x) + cos(3*x)