Depends on the resolution that clients can access I guess - giving out your data points and letting clients render the stuff might be nice (especially when having hover-text etc.), on the other side these are harder to export + performance can go down a LOT once you go past a few dozen data points (depending on the client machine of course).
PNGs should be reasonably small anyways and data likely can be cached a little bit too.
A site using highcharts would be
https://ripplecharts.com/ for example, if you want to check that out further.
