[SOLVED] Find local maximum of data files in gnuplot

Find local maximum of data files in gnuplot

I have a list of data (two columns) and I want to plot in gnuplot only the value for which my second column has a local maximum.

To do so I would like to see if the second column of the ith row is bigger than the (i-1) and (i+1)th rows.

Solution

It can be done and I was bored enough to do it. I generated the following set of random data:

Plotting the values against their position in the list looks like this:

enter image description here

You can spot the local maxima right away from the graph above. Now I can process the data points storing the two previous values (both x and y coordinates) in temporary variables, when I identify a maximum, I plot the data point:

# Select the columns of your data file that contain x and y data
# (usually 1 and 2 respectively)
xcolumn=0
ycolumn=1

plot "data" u (column(xcolumn)):(column(ycolumn)) w l, \
"data" u (column(0)==0 ? (last2y=column(ycolumn), \
last2x=column(xcolumn), 1/0) : column(0)==1 ? (lasty=column(ycolumn), \
lastx=column(xcolumn), 1/0) : lastx) \
: \
( column(0) < 2 ? 1/0 : (last2y < lasty && \
column(ycolumn) < lasty) ? (value=lasty, last2y=lasty, last2x=lastx, \
lasty=column(ycolumn), lastx=column(xcolumn), value) : (last2y=lasty, \
last2x=lastx, lasty=column(ycolumn), lastx=column(xcolumn), 1/0)) pt 7

enter image description here