Pandoc is trying to download the googleVis chart so that it can embed a static/offline copy in your document. You can work around the problem by telling Pandoc not to create a standalone document (i.e. don't do the resource embedding that's causing the problem)--add this to the top of your document, or just the text between ---
if you already have a YAML header:
---
output:
html_document:
self_contained: no
---
Pandoc shouldn't have a problem fetching resources over https. If you have time to repro the problem outside the rmarkdown workflow (i.e. on a vanilla Markdown -> HTML conversion) and find that it's still an issue, please file an issue for Pandoc: https://github.com/jgm/pandoc/issues.
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