gnuplot 2-D plotting from arrays - realtime

Question

I have to do realtime plotting of scan values of sensor. I am using gnuplot for this purpose. Till now, I am able to communicate to gnuplot from my c++ program. I tried some sample plots using a .DAT file and it is working. Now, My requirement is to plot last 5 values of sensor scan values in a single plot for comparing (that means I need to store 10 arrrays of data. 1 scan have two arrays X and Y).

What I am trying to do is to store the last 5 scan values in a column format in a .DAT file like this where x, y are are my two arrays for each scan.Then using the gnuplot command "plot 'filename.dat' 1:2" "plot 'filename.dat' 2:3" etc... Then I have to rewrite the file after every 5 scans.

X1  Y1   X2  Y2   X3  Y3   X4  Y4   X5  Y5 
2.3 3.4  6.6 3.6  5.5 6.5  8.5 5.5  4.5 6.6
4.3 4.5  6.2 7.7  4.3 9.2  1.4 6.9  2.4 7.8

I want to just confirm before proceeding wheather this is efficient for real time processing. Also Is there any command in gnuplot to directly plot from two arrays without the use of .dat files. I did not find one in my search.

Any suggestions would be helpful.

I'm not completely sure that I understand why you're question mentions 2-D arrays. It seems that you have 2 1D arrays... — mgilson

mgilson mgilson · Accepted Answer · 2012-04-12T12:43:46

Presumably, you are communicating with gnuplot via pipes. Since gnuplot is a separate process, it does not have access to your programs memory space and therefore it cannot plot your data without you sending it somehow. The most straight forward way is how you mentioned (create a temporary file, send a command to gnuplot to read/plot the temporary file). Another straight forward way is to use gnuplot's inline data...It works like:

plot '-' using ... with ...
   x1 y1
   x2 y2
   x3 y3
   ...
   e

In this case, the datafile is written directly to the gnuplot pipe with no need for a temporary file. (for more questions, about the pseudo-file '-' see help datafile special-filenames in the gnuplot documentation).

As far as this approach being useful in realtime -- as long as the gnuplot rendering speed is fast compared to the time between re-rendering, it should work fine. (I guess there are some memory issues too if your arrays are HUGE, but I doubt that would limit any real application with only 10 1-D arrays -- and if the arrays are that big, you probably shouldn't be sending the whole thing to gnuplot anyway)

gnuplot 2-D plotting from arrays - realtime

2 Answers