custom symbol overlay

[rusland]

I wanted to create a custom indicator that would compare stocks or a particular index to the sp. For example say I want a chart to have $oex and on the bottom of the chart I want a custom indicator that plots $oex / $spx ratio as a line chart. And a 20 day ma of the $oex / $spx ratio. The main issue is when I switch symbols i want this idicator to switch as well.
basically I need variable symbol / $spx line chart with 20 ma overlay on it. If any one could help please.