Hi Mac, thanks for your response. I tried it out straightaway but to no avail: whatever I got on screen (including vertical lines to the desired height) would disappear as I lifted the mouse button.
A bit of desperation-induced lateral thinking leads me to change the view/edit box setting to "control", up pops "7 main volume MSB" and then the behaviour you describe works perfectly and so I have my fix.
Now - back to my song to make musical sense with my new technique.
Can't help wondering though, just what is the purpose of the lines and blacked-out curves I can draw with view/edit set to "velocity"? I must still be missing something.
Clinton.