Need help with zenity and axel

I am trying to make a GUI for axel, but the percentage bar is not working

Code:

cat axel.sh 
#!/bin/bash

url=$(zenity --entry --text "Enter URL")

if [ "$url" == "" ]
then
 #zenity --info --title="Axel GUI" --text="User cancelled."
 exit 1
fi

#cd ~/Descargas

axel "$url" | sed 's/[^0-9.]*\([0-9.]*\).*/\1/' | zenity --progress --auto-close percentage=0

Any help is appreciated