How to change dot size in gnuplot

Either as most people answered: the file doesn't exist / you're not specifying the path correctly.

Or, you're simply writing the syntax wrong (which you can't know unless you know what it should be like, right?, especially when in the "help" itself, it's wrong).

For gnuplot 4.6.0 on windows 7, terminal type set to windows

Make sure you specify the file's whole path to avoid looking for it where it's not (default seems to be "documents")

Make sure you use this syntax:

plot 'path\path\desireddatafile.txt'

NOT

plot "< path\path\desireddatafile.txt>"

NOR

plot "path\path\desireddatafile.txt"

also make sure your file is in the right format, like for .txt file format ANSI, not Unicode and such.

I don't have your data so I made some up (it always helps to be helpful if some useful mock dataset is given...):

0   0   0   0
200 1000    1200    1500
400 4000    7000    9000
600 7000    15000   18000
800 12000   23000   25000
1000    18000   33000   40000

Based upon your code, I tried

reset
set terminal png
set xlabel "Square matrix size"

set ylabel "Time (in milliseconds)"
set xrange [0:1200]
set yrange [0:50000]


set title "Lower Triangular Matrix"
set key reverse Left outside
set grid
set output 'matrixlt.png'
set style data linespoints
plot "matrixlowertriangle.dat" using 1:2 lt 1 lw 2 smooth bezier title 'MatPro', \
     "matrixlowertriangle.dat" using 1:3 lt 2 lw 2 smooth bezier title 'C#' , \
     "matrixlowertriangle.dat" using 1:4 lt 3 lw 2 smooth bezier title 'C++' , \
     "matrixlowertriangle.dat" using 1:2 with points title "", \
     "matrixlowertriangle.dat" using 1:3 with points title "", \
     "matrixlowertriangle.dat" using 1:4 with points title ""

and got

Is that closer to what you want?