gnuplot - Select bin center in histogram

Question

I'm plotting a histogram with the commands:

n=3
max=0.5
min=-2.5
width=(max-min)/n
hist(x,width)=width*floor(x/width)+width/2.0
plot "< awk '{if (\$1<3.9 && \$1>3.8) {print \$0}}' /path_to_file/" u (hist(\$2,width)):(1.0) smooth freq w boxes lc rgb "black" lt 1 lw 0.5 notitle

which gets me this:

histogram

Apparently, I'm guessing based on the data file, gnuplot is deciding by itself to choose the intervals [1.0:0.0]-[0.0:-1.0]-[-1.0:-2.0] which makes the bins centered in [0.5:-0.5:-1.5]

I need those bins to use the intervals [0.5:-0.5]-[-0.5:-1.5]-[-1.5:-2.5] which would make the bins centered in [0.0:-1.0:-2.0]

How can I accomplish this?

Hi @mgilson, no I want gnuplot to actually use those intervals I wrote and not the ones it appears to be using by its own decision. How can I tell gnuplot from what position to start counting to generate the intervals with the width I set? — Gabriel
So you want to transform x -> x-0.5? Try: hist(x,width)=width*floor((x-0.5)/width)+width/2.0 — mgilson
mmm I'm not entirely sure of what that is doing but it's not what I need. I need gnuplot to center the bins around 0.0, -1.0 and -2.0 (with a width of '1') instead of how they are centered by default which is in 0.5, -0.5 and -1.5. I want gnuplot to count the number of objects inside each interval [0.5:-0.5]-[-0.5:-1.5]-[-1.5:-2.5] and not what it is doing right now which is counting them in the intervals [1.0:0.0]-[0.0:-1.0]-[-1.0:-2.0]. — Gabriel

Sunhwan Jo Sunhwan Jo · Accepted Answer · 2012-12-15T21:47:25

The interval in your histogram is determined by the function hist. hist function reduces a range of numbers into a single value, so gnuplot can plot in a bar plot centered at the value returned by hist function.

In your original post, hist(x,width)=width*floor(x/width)+width/2. Hence, you will get hist(x,width)=0.5 for width=1 and 0 <= x < 1. Eventually, you will get a bar plot centered at 0.5.

Now, if you want to set your interval as [-0.5:0.5], [-0.5:-1.5], and so on, you may want to change your hist function so that it returns 0.0 when width=1 and -0.5 <= x < 0.5.

width=1
hist(x,width)=width*floor((x + width/2)/width)
plot "< awk '{if (\$1<3.9 && \$1>3.8) {print \$0}}' /path_to_file/" u (hist(\$2,width)):(1.0) smooth freq w boxes lc rgb "black" lt 1 lw 0.5 notitleenter code here

gnuplot - Select bin center in histogram

2 Answers