Arduino osciliscope

I'm going to be building the osciliscope project derived from that in a book (practical Arduino). However I'm unsure of a few things relating to my planned design 1) if the outer metal of the usb socket touches the metal housing, what will happen?

2) 2 sockets will be connected to prescalers, so I can handle readings above 5v, I believe I can use a switch to select the prescaler (ie 1/4, 1/10 or input voltage . 3) due to only using short leads, I plan on using DMM leads as they are not expensive (cro leads are at least $40nzd over here) which unlike cro leads are unshielded.

4) does any pc software that can take the data stream from the arduino appear on the pc screen like an old fashioned osciliscope, the arduino based circuit pc programs seem to (on screenshots) generate a plot like as if it's drawing the plot on a sheet of unrolling paper when I'd like to have a classical looking screen as I'm planning on only having two analog inputs.