There are a few ways to add a script like Plot to a WordPress site. The simplest is to use a Script Manager like Tracking Script Manager, this method is detailed below. Alternatively, if you're more technical, you can use wp_enqueue_script or just insert the script in your header file directly.
Note: This method of installation is for WordPress.org blogs. Unfortunately, WordPress.com does not allow scripts to be included on pages. Consider Tumblr for free blog hosting instead.
First, open your WordPress blog dashboard in your browser. Next, hover over 'Plugins' from the left sidebar and click 'Add New'.
Search for 'Tracking Script Manager' in the WordPress plugin library. Install the plugin on your WordPress site.
After Tracking Script Manager is installed in your project you need to click the 'Activate Plugin' button.
Click the 'Tracking Script Manager' plugin in your sidebar. Note: The plugin doesn't actually track anything itself but lets you install Plot's script on your site.
Copy your Plot script from here. You can also find the script in the settings page of your Plot project.
Paste your Plot script into the Tracking Script Manager and give it the name of Plot. The other options can be left as default. Then just click 'Add Script'.