Combine several images into one mosaic with Python and PIL

I have a sub directory data with multiple jpg picture files. The files are named in a way that they are sorted automatically by the os. In my case this is YYYYMM-xy.jpg where YYYY is the year and MM is the month. This is because for this project I made one picture every month over one year. The sub string -xy is extracted by the Python program and is rendered as text onto each image.

 

With the attached Python script I can combine these eight images into one. [mosaic.py]