At the end of last year I created a visualization of 500 miles of running in 2014[1] and posted it to a few sites. A lot of people were interested and wanted to make their own but the code was... not good. So I decided to turn it into a mini-app that anyone could use. At the moment this is barebones MVP. Just raw GPX/TCX upload, no error handling or syncing with third parties etc.http://www.gpsheatmaps.com/
[1] https://www.flickr.com/photos/dachande663/15965476529/