Great question — answering as the UphillTech team:
Here are a few search engines that do seem to offer the options you are looking for (all of them have been mentioned on this site before): Momondo requires you to pick a date but then displays a graph with prices for a whole month. Adioso offers several ways to flexibly specify dates and also accepts a whole country or region as destination. Kayak's explore functionality is very flexible both in time and in space. In fact, ITA's matrix also has most of what you require: flexible date picker, ability to specify multiple airports one-by-one, etc. Compared to Adioso, you still need to do some work by varying the length of stay or the airports but that's almost as flexible as it gets,...
Full answer by Relaxed: https://travel.stackexchange.com/a/39311 (CC BY-SA 3.0).
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