Avoid error "Modification of non-creatable array value attempted"
trying to access unallocated positions.
($opt{width} - $lenval - $len) / ($maxval - $minval); # bar multiplication
my @barmark;
-if ($opt{markers} // 1) {
+if ($opt{markers} // 1 and $size > 0) {
sub orderpos { (($order[$_[0]] + $order[$_[0] + .5]) / 2 - $minval) * $size }
$barmark[ (sum(@order) / @order - $minval) * $size ] = '='; # average
$barmark[ orderpos($#order / 2) ] = '+'; # mean