After much trial and error, the below creates incremental, decimal values in a bash for loop for use in the imagemagick utility, convert.
j=56
k=4.5111
for i in *.jpg
do
p=$(echo "$j + $k" | bc)
convert $i -draw "line $p, 581 $p, 514" "${i%.*}.jpg"
j=$(( $j + 1 ))
done