It's worth noting that Hello Charts and all of the other chart blocks mentioned use the Chart.js Javascript library in the front end, which weighs in at 184kb. This is pretty heavy and could really impact performance on sites that already have a lot of Javascript running.

I'd like to see a chart block that uses an SVG-based charting library to render the chart in the editor and save the markup in the database, which would then require little to no client-side Javascript for visitors.