postMessage() function

I have the diagram below:

The code for the diagram on the html subpage:

Title ](https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.4.0/Chart.bundle.min.js">)

I just don’t know how to send information about variables (a, b, c, d, e, f, g) from the page using the
postMessage() function,
could anyone help?