How to draw a 2D line plot using C#?

It depends on what UI library you want to use: System.Windows.Forms or WPF.

WPF is much more powerful and easier to use (when it comes to plotting, I don't say in general). There are many approaches. I thing you need to use canvas and put graphical objects on it. In this case, you don't have to care about rendering of graphics, you only support properties of your objects. Quality and performance are very good as it all based on ActiveX, all graphics is anti-aliased, with full support of transparency (alpha channel). You can automatically scale you graphic, fit it with preserved aspect ratio or otherwise, perform affine transformations and a lot more. One can develop plotting components using set of objects composed out of graphical primitives (such as Rectangle, Ellipse, Besier curve, Text, etc.).

In contrast to that, System.Windows.Forms needs rendering you can make in Control's event handler Paint (or overridden method OnPaint). You can create custom control or use Panel or a control derived from Panel. The event argument carries Graphics object used to call its drawing methods. This rendering functionality relies in Windows message WM_PAINT. As in native Windows GDI and GDI+ one needs to define rendering procedure called every time WM_PAINT is sent to a control. The event is not sent until control is invalidated. It's overloaded method Control.Invalidate comes in parameter-less variety used to invalidate a control as well as with a parameters used to invalidate some sub-set of the scene: Region or Rectangle. This way, modification of the graphics is done by modification of some data used in rendering followed by the call to Invalidate. In contrast to WPF, drawing primitives is always done in pixels, however pixel measurements are of the float type: due to (optional) anti-aliasing sub-pixel accuracy of the measurements of graphics primitives is used. This library also provides sufficient capabilities for development of plotting components. It requires more understanding of rendering mechanisms and generally provides lower performance compared to WPF.

Further questions are welcome.

Good luck,
--SA