>Any computer or PDA with decent sound hardware should suffice for a
>working, no-cost, software-only approach. Just connect your analog
>input to line-in (adapt level and impedance as needed), and record
>your signal as an audio stream, which you can later transform into
>whatever kind of plot or display you like. It won't be spectacularly
>accurate, granted, but it'll still outperform a paper strip plotter on
>all practical counts.
>
For frequencies from 20Hz to 20kHz or so you're right and not only because the typical paper strip plotter's performance in that range is rather poor.
(Most (all?) PC soundcards ar AC input only.)