Imagemagick based automation tool

I needed a tool to generate some images for MRTG index page. Imagemagick has some programs to create and convert images. Very useful guide is on Imagemagick site. Look at this:

for STRING in MySQL1 MySQL2 Loghost Backup; do
  # Variables "invented" experimentally
  A=3; B=70
  # Length of string
  X=${#STRING}
  # Where to start to draw the label
  Y=$((A*X+B))
  # Imagemagick tool with PostScript command to rotate label
  convert -size 20x120 xc:white  \
    -font /usr/share/fonts/truetype/ttf-dejavu/DejaVuSerifCondensed.ttf \
    -pointsize 16 \
    -fill black \
    -draw "rotate -90 text -$Y,15 '$STRING'" /tmp/${STRING}.png
done

Some experiments gave me variables for equation to center the label vertically. I can't do it in PostScript.