The attached heat graph is one that I tend to use often and it is always much appreciated in its simplicity. When it comes to getting an overview over many products/customers/markets etc over time it tends be clear when several other types of graphs quickly gets cluttered. Simply scrolling down the rows, even when you have many rows, will give you a good picture of what is going on in terms of e.g. customer activity, usage patterns etc.
Here are a few areas where I have found it useful to include the graph; Spotting competitor market activities in different countries. It is very easy to see where someone has entered or ceased their activities. Another are is to understand usage of different services (originally the purpose of the picture to the right). A third is to monitor changed procurement patterns among customers. Of course these are just some examples to fuel your inspiration.
How to do it? It is quite simple. It is a variant of a pivot table. Blank spaces are assigned as values in the expression, then the background colour shows the value. If you need it to keep blank periods when only selecting one line, you need to fill in missing values with 0 for that dimension in the script. Seldom necessary though as value comes from watching a large number of values, rather than a selected few.
The QVD is attached. I hope it will prove useful for you!