Please help with Threading

[Jurgens de Bruin]

This is my first script where I want to use the python threading module. I have a large dataset which is a list of dict this can be as much as 200 dictionaries in the list. The final goal is a  histogram for each dict 16 histograms on a page ( 4x4 ) - this already works. 
What I currently do is a create a nested list [ [ {}  ], [ {} ] ] each inner list contains 16 dictionaries, thus each inner list is a single page of 16 histograms. Iterating over the outer-list  and creating the graphs takes to long. So I would like multiple inner-list to be processes simultaneously and creating the graphs in "parallel". 
I am trying to use the python threading for this. I create 4 threads loop over the outer-list and send a inner-list to the thread. This seems to work if my nested lists only contains 2 elements - thus less elements than threads. Currently the scripts runs and then seems to get hung up. I monitor the resource  on my mac and python starts off good using 80% and when the 4-thread is created the CPU usages drops to 0%. 

My thread creating is based on the following : http://www.tutorialspoint.com/python/python_multithreading.htm

Any help would be create!!!