I was looking for this too. Hoping to be able to click on any given station and see the graph history for it. Is all old data discarded after each refresh? Ah, I see that there is a history you can view for your own data at lease.
On gmcmaps each contributor station decides whether their data is available for download by others. Stations can download their own data even if they don't make it available.
If they agree, a button appears in their gmcmap public pop up profile on the map. (Click any pointer and one of these will appear).
Contributors can also agree to receive email and another button appears on their profile. They can also provide other information, details sometimes not entered, like counter type and station name. It's voluntary.
Not all stations that appear on the initial map are active contributors. Click the 24 hour time range to see the active ones. In some areas, only a few are active in the last 24 hours.
I don't know about rules for removing data. My impression is that any data that was entered on gmcmaps is still there. There was data I sent with an earlier counter from a year ago. You can delete your own data and I removed that dataset.
I use a spreadsheet (Libreoffice) to make graphs from downloaded CSV data.
So far I am limited to 500 data points in a download. The time spacing of the data points is set by the station so for mine that's about three days worth. What it looks like: