# Plots only logarithmicly spaced samples

I have a data file with many entries, order of 10000. I want to plot them all with lines, and a small subset of them with points and error bars (can not plot error bars on 10000 samples). Plotting each N is easy done using each nth point=N, but I am using a semilogarithmic axis, so I want to plot each 10^N (even better 2^N).

Right now I preprocess the data file and generate another one with the interesting samples subset, is there a way to make pgfplots do that?

### EDIT

This is a minimal working example:

\documentclass[10pt,a4paper]{article}

\usepackage{pgfplots}

\begin{document}

\begin{tikzpicture}
\begin{semilogxaxis}
\addplot+ table[col sep=comma, x=x, y=y, each nth point=100]
{aaa.txt};
\end{semilogxaxis}
\end{tikzpicture}

\end{document}


File aaa.txt is something like this:

x,y,var
1,1,0.7257899338880273
2,2,0.5103550634511824
3,3,0.8469400089552953
4,4,0.8351651602434369
5,5,0.07359721861492374
6,6,0.3107395762944515
7,7,0.9151522020717112
8,8,0.4967609141914707
9,9,0.2218284814240239
10,10,0.7166461578126254
...


I generated the coordinates with Ruby's script 1.step(1024) { |i| puts "#{i},#{i},#{rand}" }. With using nth point=100 it plots only at 100, 200, 300... I tried replacing it with the x filter suggested but it keeps plotting the whole data file (it does not say on command line that it is skipping points because a filter, as it does for using nth point).

-

Replacing each nth point=N with the following should work :

x filter/.code={\pgfmathparse{divide(#1,equal(#1,floor(#1))}}


Some explanation : x filter allows, as the name says, coordinate filtering. The code is provided the coordinate as #1 (or its log in case the axis is logarithmic, which is the case here), and performs some computation on it. If the result is a number, it is used as the coordinate, if not, it is discarded.

Here we want to discard the cases where log(x) isn't an integer, so we test equality between log(x) and Ceil(log(x)). If they aren't equal, the code performs a division by zero, which results in inf and is discarded. If they are equal, on the other hand, dividing log(x) by 1 gives back log(x).

Be sure to add log basis x=2 in your semilogxaxis environment options if you want to filter powers of 2.

Now that's for the theory. However, if you try the above code with your example, it won't work, because pgfmath computes the logarithm in base 2 wit poor roundings, e.g. log2(8)=2.99997. The following code does work. Please keep in mind that it still relies on approximate calculations, and as such, that it may fail for bigger values.

\documentclass[10pt,a4paper]{article}

\usepackage{pgfplots}

\begin{document}

\begin{tikzpicture}
log basis x=2,
x filter/.code={\pgfmathparse{divide(#1,(equal(2^(#1),2^(round(#1)))))}}
]

Sorry for the late answer, I had to fix more important things and I left this behind for few days. I tried your suggestion, read the doc about x filter, but I could not get it work, it plots always all the points. I added a MWE in my question. – Claudio Nov 19 '12 at 14:02