Depending upon the number of data points you are representing, images may take less bandwidth and transfer time than the data itself. For example suppose you have a chart showing network latency; if you sample once a minute and show a month's worth of data you could easily transfer around 500KB for the raw data (assuming 12 bytes of json per sample), while an image file would be 20KB. If you have a dashboard full of images like this, the time to download all that data could become pretty significant to the end-user.
I see your point, but that's a somewhat contrived example: If you display per-minute data for an entire month you won't have the horizontal resolution (43200 to 44640 data points) to plot them all!
If you want zoomable charts to allow for increasing resolution, then the chart lib should ask the controller for a specific resolution and range on demand.
Even then, 500KB of numeric JSON data compresses really well (I tested with 1 month of real-world per-minute data: 92KB).
