Use Google Chart API for your web apps

Google Chart API is a tool added to Google Spreadsheets which allows you to build a URL that will dynamically create a chart for you. Before the public release this service was used internally for Google Finance, Google Video, and beyond. Now, anyone can use it!

With Google Chart API you can create more types of charts: columns, bars, lines, pie, scatter, add labels and a legend. Just select the columns you want to plot and click on the new chart icon. After inserting the chart in the spreadsheet, you can save it as a PNG image or edit it.

The charts are rendered as SVG in Firefox/Opera and VML in Internet Explorer, so they don't require plug-ins. As usually, Opera is not officially supported, so you'll find things that don't work as expected.

Here are some sample charts:

Line Chart

Bar Charts

Pie Charts

Venn Diagrams

Scatter

Solid Fill

Linear Gradient

Linear Stripes

Legend

Multiple Axis Labels

Fill Area

And there are many more options available.