Populate or change the selection options in one field based on selected option in another field in the CP I'm looking to change a select fields options with the onChange event from another select fields chosen option. Obviously this can be done with js on the frontend as per this post but has anyone done this for the control panel? I imagine it will need to be done via a fieldType plugin, but wondered if anything already exists for this type of functionality before I attempt to build something.
I would write a custom fieldtype (http://buildwithcraft.com/docs/plugins/field-types) with two selects. Makes sense to make it as one field if they are dependent of eachother.

Just store the field as json (AttributeType::Mixed) and inject your own javascript in the fields getFieldHtml()