Charts are an indispensable part of any data visualization work. People can grok visual representation of data easily than a textual variety. Today there are a huge number of free chart libraries available to display visualizations on websites. Below is a small list of  some of the libraries available for displaying charts using php.

1. Visifire
output format: Silverlight
license: GPL 3.0 / Commercial

Visifire is a set of open source data visualization components – powered by Microsofts Silverlight technology. Visifire  can be used in both WPF & Silverlight applications. Using the same API, charts in both Silverlight & WPF environments can be created within minutes. Visifire is independent of server side technology. It can be used with ASP, ASP.Net, PHP, JSP, ColdFusion, Ruby on Rails or just simple HTML. The website also includes a chart designer for you yo play with. The components also support excellent animations.

Features:

• Visually Stunning Animated Charts
• Single API to both Silverlight & WPF
• Embed into Desktop or Web Applications in minutes
• Real time Charts / Live Update
• Tiny footprint
• Enterprise grade features
• Microsoft Expression Blend compatible
• Dual License – Open Source & Commercial

2. PHP/SWF Charts
output format: Flash
license:Free / Commercial

PHP/SWF Charts is a simple, yet powerful PHP tool to create attractive web charts and graphs from dynamic data.

Use PHP scripts to generate or gather the data from databases, then pass it to this tool to generate Flash (swf) charts and graphs. Any other scripting language (ASP, CFML, Perl, etc.) can be used with XML/SWF Charts (the XML version of the same tool.)

Some features:

• Web charts and graphs from dynamic data
• Live and interactive chart updates without reloading the web page
• Clickable charts, and drill-down
• Animated transitions
• Printable charts
• Simple and flexible chart generation
• Quality Flash graphics
• Supports PHP 3, 4, and 5
• Supports unicode text to display special characters and any language

3. pChart
output format: Image
license:GNU General Public License

pChart is a PHP class oriented framework designed to create visually appealing charts.  Data can be retrieved from SQL queries, CSV files, or manually provided. This project is still under development and new features or fix are made every week.

4. Open Flash Chart
output format: Flash
license:LGPL

Open Flash Chart is a wonderful program for creating charts in Flash. The library supports bar, line, area, and pie charts. It also includes server side classes to help generate the data file used as input. At present there are classes for PHP, Perl, .NET, Java, Ruby (and Rails), and Python.

Implementation  is quite easy.  All you have to do is copy the Open Flash Chart SWF file to your web server, then start embedding flash charts into your HTML and point to static or dynamic data on your server.

5. amCharts
output format: Flash / Image
license:Free / Commercial

One of my favorites. AmCharts is a toolbox of Flash charts for your websites. AmCharts can automatically extract data from simple CSV or XML files, or they can read dynamic data generated with PHP, .NET, Java, Ruby on Rails, Perl, ColdFusion, and many other programming languages.

Key features:

• Animated or static
• Value axis can be reversed
• Linear or logarithmic value axis scale
• Predefined or custom bullets
• Custom description for any data point
• Clickable columns/bars (can be used for drill-down charts)
• Values can be converted to formatted durations
• Gradient fills
• Value indicator plug-in
• Axis values inside the plot area

Common amCharts features:

• Platform-independent
• Can be exported as an image
• Can be controlled via JavaScript
• Settings and data can be passed directly from an HTML file
• Custom image or Flash (animated) backgrounds
• Unlimited labels
• Rotated labels and axis values
• Data can be automatically reloaded with a preset frequency
• Format any balloon text

6. ezGraph component
output format: SVG / Image
license:BSD license

eZ components are an enterprise ready general purpose PHP components library by eZ systems. As a set of quality independent building blocks for PHP application development, eZ components help you speed up your php development.
The Graph component  enables you to create line, pie and bar charts. The output driver mechanism allows you to create different image types from each chart, and the available renderers make the chart output customizable from simple two-dimensional charts to beautiful three-dimensional data projections.