You’ll find more off-the-shelf examples included when you download Protovis.
(Most of the examples you'll see in the documentation use method chaining to make specifications more concise. This means that the method to set a property returns a reference to the mark itself, allowing you to set multiple properties in a single statement. If you prefer a more traditional verbose style, you can use that instead.)
Breaking it down:
1. The first <script> element loads Protovis. You need to download protovis.js and install it in the same directory as the HTML file you are editing, or update the path accordingly. The precise name of this file depends on which version you download; if you're just getting started, you probably want to use a development version, such as protovis-d3.2.js. You can place this <script> element anywhere you like, as long as it is loaded before you try to visualize anything. Inside the <head> element is a safe bet.
3. The example visualization uses a panel that is 150 pixels wide and 150 pixels tall. The SVG element is inserted into the document at the same location as the script. The words “Hello, world!” appear centered in the canvas.
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