Seasonal variation in the numbers of Phaonia subventa

I have written a little script to help me create plots showing the seasonal variation in the numbers of various species of fly I have collected.

Most species show a large peak in numbers in late spring or early summer with none or very few in winter.  The Muscid fly Phaonia subventa is unusual in showing fewer flies in the summer than in the winter.  I had noticed that it was one of the most common woodland flies in winter (I have seen them on Christmas day morning, sunning themselves on tree trunks when the temperature was below 5C) but I hadn't realised, until I saw the above plot, that they seem to almost completely disappear during the summer months. This might be a consequence of the dry summers we have had in the south of England in the past few years.

My script is still a bit primitive (it is written in Gnu Awk!) and I intend to tidy it up and extend it (and maybe rewrite it in Rust).  But if you are interested you can find a copy in the Github repository where I store the catalog of my fly collection (https://github.com/tristrambrelstaff/flies).  The script is mplot.awk.