The retrieval of data from the server can be either by data-push or data-pull technologies. Since these topics are outside the scope of this book, we will limit ourselves to basic Ajax techniques. With Ajax, we can update a chart without the need to reload the whole page. This feature can be used to update charts when new data becomes available on the server.

In the following example, we will use Ajax to get data and update an already initialized chart at the click of a button. Contrary to the previous example, we will not mix our PHP and JavaScript code in this case, but rather will keep them in two separate files for better maintainability and readability.

We will use the same database and the same data as the previous example for the purpose of elaboration. Initially, our chart will be loaded with some random data. This can be considered as an initial state with the chart showing old data. As soon as an event occurs (in this case, a click), the client (browser...