Hi Shao-Yu Chen,
ASDM displays data that comes from the ASA via the asdm_handler. The ASA pushes (or should push) data every 10 sec. Could you please open a browser and point to https://x.x.x.x/asdm_handler. Run that in parallel to the ASDM. When there is a break in the ASDM graph, check the browser input to see whether there is a similar break there. The timestamp of each data push looks something like this:
The last two digits of the 2nd piece of data are the seconds and they should be incremented by 10 with every chunk of data:
If there are breaks in the asdm_handler data, it's an ASA issue. If you don't see a break around the time there's a break in ASDM's graphs, please check the whole browser output to see if you see a break in the browser as well.
Either way, I would recommend openning a ticket with TAC to further investigate.