Please check that the Plotalot Plugin is installed, and that it is enabled in the Joomla Plugin Manager. The Plotalot Plugin must be installed separately from the component and then enabled in the Plugin Manager. There is no free version of the plugin, it is a paid download.
If the Plugin is installed and enabled, but still not working properly, view your article in HTML mode. Some wysiwyg editors make a real mess of things. This is a real example:
{plotalot id="39<br />}"If you can't get the article to save cleanly, you might also need to disable Article Text Filtering in Joomla Global Configuration.