Download the fotherplot plugin and you will get a directory called "fotherplot" when you unzip the downloaded file. Upload the "fotherplot" directory to your WordPress plugins directory (e.g. /wp-content/plugins/)
Activate the plugin through the 'Plugins' menu in the WordPress dashboard.
Do you already have some data that you want to chart contained in a custom field? If so, go to the Fotherplot settings page and specify the custom field name in the dropdown. If not, decide what the name of the custom field that you want to plot is going to be and create a couple of posts (with different publish dates) first.