I think Abdul wanted to say, that intensity modulation can be done by remaining on a spot for a time (e.g. when the oscilloscope has no Z input). Or, the brightness can be controlled by controlling how fast the X and Y signals change.
This can be accomplished in two ways:
- use many-bits DACs and make up the lines from dots
- go analog.
The scope-clock uses a 8-bit DAC = 256 steps, and when the beam is on, it moves really step-by-step.
The VECTREX circuit used a real integrator (an opamp), with capacitor feedback shortable by a analog switch to enable fast changes of the output signal.
However, both solutions employed also the Z signal. But maybe if the trace moves slow enough during the "drawing" phase and fast enough in the "moving" phase, the "moving" might get unnoticed.

Needs to be tried with a particular oscilloscope.

Jan Waclawek