I guess that by plotting you are thinging about a chart (and not something more sinister :) )
There is one project here on CP that may suit you:
A flexible charting library for .NET[
^]
[Updated for c]
Cairo is a 2D graphics library with support for multiple output devices
http://cairographics.org/[
^]
On the
Examples page[
^] - you will find links to various charting examples/implmentations
If you don't mind using GTK+ have a look at
GTK/Cairo Line Graph widget[
^]
Best regards
Espen Harlinn