Thats a very tough problem to solve. Just brainstorming about how to solve it reliably, I think the hardest part would be to get the axis tick mark values. How would you know, or get a program to detect what the graph's axis gradations are? Just for the sake of argument, if they were constant, and you could hard-code those into the program, you might be able to use one of the graphics modules, like Imager or ImageMagick, to do a pixel-by-pixel deconstruction of the image. Since most graphs have a constant background color, and a single graph color, you probably could analyze that data for peaks and lows. I'm not suggesting the following code is a solution, but it shows how to do a pixel-by-pixel deconstruction.

#!/usr/bin/perl 
use strict;
use warnings;
use Imager;

# assumes a rgba image ( with alpha )
# otherwise use rgb instead of rgba

my $image_source = shift @ARGV;

my $image = Imager->new;
$image->read( file => $image_source )
  or die "Cannot load $image: ", $image->errstr;

my $width  = $image->getwidth();
my $height = $image->getheight();
print "Image dimensions: height = $height, width = $width\n";

# only grab some pixels on line 10
# for the whole image iterate over $width and $height

my $x = 10;
foreach my $y ( 10 .. 15 ) {
    my $color = $image->getpixel( x => $x, y => $y );
    my ( $r, $g, $b, $a ) = $color->rgba();
    print " shade = $r, $g, $b\n";
}
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