The CPC+ and GX4000 contain an analog joystick port (additionally to the normal Digital Joystick port).
Technical
The connector is similar (but not compatible) as PC/Soundblaster joystick ports, for details on the pin-outs (and differences between CPC and PC joysticks), see:
For info on reading the analog inputs, see Arnold V Specs Revised. The buttons are accessed same ways as for Digital joysticks (ie. as part of the keyboard matrix).
Analog CPC+ Joysticks
Were there any manufactured...?
CPC+ Games with Analog Joystick Support
Were there any released...?