Why gnuplot rounds data in column of vertical axis?

Question

My old script worked fine years ago.

set terminal png background "#ffffff" enhanced fontscale 2.0 size 1800, 1400 
set output 'delete.png'
w=1
x=1  
z = 60 
y=2
plot 'plot.in.tmp' using (column(x)/z):(column(y)) axis x1y1 with lines 
exit gnuplot
reset

Now result in graph with only rounded integer points in y(vertical) axe. I dont understand why. Example data in file:

0     -0,00        0,5    570,2    11,98     -0,121      0,000        9,6 
5     -0,00        0,7    570,2    11,97     -0,002      0,012       13,2 
10     -0,00        0,9    570,3    11,98     -0,004     -0,000       16,1 
15      0,24       35,9    570,4    11,96      0,001      0,000       18,4 
20      0,56       87,0    570,1    11,99     -0,001     -0,000       20,5 
25      1,03      173,5    570,4    11,97     -0,000      0,000       23,2 
30      1,61      296,4    570,3    11,96      0,002      0,000       12,4 
35      2,17      422,6    570,2    11,68      0,004      0,000        8,8 
40      2,81      571,6    570,2    11,37      0,010      0,001        7,5 
45      3,52      752,3    570,3    11,26      0,015      0,000        7,1 
50      3,97      905,0    570,2    11,69      0,075      0,006        7,4 
55      4,36     1048,4    570,1    11,36      0,081      0,001        8,6 
60      4,59     1156,8    570,2    11,22      0,087      0,001       10,7 

Result graph:

Answer 1

The answer given by theozh is correct, but it does not point out the unfortunate lack of standardization about how different operating systems report the current locale setting. For linux machines the locale strings are less human-friendly. For example instead of using something generic like "french", they subdivide into "fr_FR.UTF-8" "fr_BE.UTF-8" "fr_LU.UTF-8" etc to account for slight differences in the conventions used in France, Belgium, Luxembourg, etc.

I cannot tell you the exact set of locale descriptions on your machine, but here is what works for me on a linux machine:

set decimalsign locale "fr_FR.UTF-8"
w=1
x=1  
z = 60 
y=2
plot 'plot.in.tmp' using (column(x)/z):(column(y)) axis x1y1 with lines