It's not a case of how often the chart is updated.It's a case of how often users push CTRL-F5.Charts are complex images that don't compress well without bad artifacting.A single one could have 10 X the data load compared the the image heavy front page on SR.It could easily double the data bandwidth requirements for the site, or even more if people viewed the chart only 20% as often as they viewed the front page.If it was any higher than that, the site would crawl to an absolute halt.So yes, it's purely a bandwidth issue. There's a limited amount of bandwidth a tor site can push out, and once it's reached, you're fucked.