Your quality score history is available in the second tab of the main dashboard section.

It can be found at the account level, campaign level and ad group level.

You can single out specific elements to be displayed in this area by clicking the left button besides campaigns, ad groups or keywords:

This is how you track your progress overtime and see if your optimizations are working or not. It doesn't only show quality score variations, you can use the top drop down menus and select which metrics you want to compare.