Thanks for the pointer to the PDL package, tye. Here is the polynomial fit I did above converted to use piddles. But as you say, polynomials may not be the best thing to use. 
use strict;
use PDL::LiteF;
use PDL::Fit::Polynomial;

my $xp  = pdl (62.6,    62.8,    63,    63.3,    63.5,    63.7,    64)
+;
my $yp  = pdl (7.476,    7.5219, 7.5464, 7.5516, 7.5504,    7.5372, 7.
+518);

my ($yfit,$coeffs) = fitpoly1d($xp,$yp,3);
my $diffp = $yp - $yfit;
              
my @theta = list $coeffs;
print "Coefficients: ",join(",",@theta),"\n";
my $c = $theta[0];
my $b = $theta[1];
my $a = $theta[2];

print "\nComputed results:\n";
my $xtop = $xp->nelem - 1;
for my $i (0..$xtop) {
   my $x = at($xp,$i);
   my $y = at($yfit,$i);
   my $ytrue = at($yp,$i);
   my $diff = at($diffp,$i);
   print "$x\t$y\t($ytrue)\t$diff\n";
}

## Solve for the derivative being 0.
my $maxx = -$b/(2.0*$a);
my $maxy = ($a*$maxx + $b)*$maxx + $c;
print "\nPeak point:\n";
print "$maxx\t$maxy\n";

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In reply to Re: Re: mathsy question: finding max of curve from discrete points (two steps) by tall_man
in thread mathsy question: finding max of curve from discrete points by Anonymous Monk

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