Similarity Matrix and Heat Chart

Suppose you have the following dataset which contains which contains (1st tab) a list of items purchased by a given user, (2nd tab) a mapping which maps the item_id to the item name and price, and (3rd tab) a matrix that formats data from sheet 1 into a matrix with users in rows and the number of each item_id purchased.

Using this data, can you create a similarity matrix for each item? Then can you create a heat chart that visually displays the similarity matrix?

Note: The purpose of doing this exercise is to explore ways of analyzing data that could be generalized by a group (think clustering).