TimeDataPlot is a free and easy to use opensource asp.net control for plotting data against time. The control is free for use and modification. When you release a modified version it would be nice if you mention the original. The download contains the source for an example like the one below.

Syntax Example:

<%@ Register TagPrefix="Stickler" TagName="TimeDataPlot" Src="TimeDataPlot.ascx" %>

[...]

ExamplePlot.PlotTitle = "Random Plot Example";
ExamplePlot.ImageAlt = "Random Plot Example";
ExamplePlot.DataSource = myDataView;
ExamplePlot.TimeColumnName = "time";
ExamplePlot.ValueColumnName = "value";
ExamplePlot.PlotPaddingLeft = 60;
ExamplePlot.ImageWidth = 450;
ExamplePlot.ImageHeight = 250;
ExamplePlot.DataBind();

[...]

<Stickler:TimeDataPlot id="ExamplePlot" runat="server" />

A real example where TimeDataPlot is used can be seen here, where the 24h data of my brother´s weatherstation is shown.