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 (though still unknown which of the eight ADC0..ADC7 inputs are mapped to the four X1,Y1,X2,Y2 pins...?)
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...?