Edouard,
I don't believe I've experienced this sort of problem since upgrading to Yosemite, but I've only been looking at memory data from Activity Monitor and swap file usage from iStatMenu memory stats. The highest memory usage I have ever seen for WeatherCat is 530MB but it generally runs around 450-500MBs. Currently, the 'Cat on my machine has been up continuously for 6 days, 3 hours and is sitting at 496MB (via Activity Monitor) with 12 custom gauges open, CustomWeb and WU, CWOP, Weathercloud and PWS uploads running (if that counts for much in memory usage).
It appears you are running some sort of script to make your usage graph, so perhaps I could try that for comparison?
BTW, I'm on a late '12 iMac with 16GB.
Cheers, Alan