Piwik already tracks everything needed to give a good overview over evolution of performance of a store.

What is missing is, doing some simple math with the values and show them beside the already available ones in ecommerce evolution over the period graph.

These values often are used to get a quick overview and are actually (2.13b1) missing in the graph:

• purchased products/Order (average number of purchased products per order)
• revenue per visitor
• ratio abandoned carts/ordered Carts
• percent of orders without shipping costs (shipping = 0€, the store owner has to pay for it..)
• percent of orders with discount
• average discount (total revenue/total discount)

and in addition not available within piwik today, but very very important for this overview:

• cost per revenue, see Analyse costs and revenue for a single visitor/a whole campaign #4458