SiteMetrics currently uses the Google Chart API. The problem with this is that this API is closed-source (proprietary software). It's sub-accompanying APIs by Google, the Infographics API and the Image Charts API happen to be deprecated:

Source: https://developers.google.com/chart/infographics/
While the separate Google Charts service is actively maintained, the infographics service is now deprecated. For now, images created via this API will continue to serve, but please do not create new infographics using this service.

Source: https://developers.google.com/chart/image/
While the dynamic and interactive Google Charts are actively maintained, we officially deprecated the static Google Image Charts way back in 2012. This gives us the right to turn it off without notice, although we have no plans to do so.

We should consider changing to something FOSS (Free Open-Source Software), such as Chart.js or D3.js. I'll submit this and in a comment/edit, I'll add a comparison between the two software.