Find queries causing a lot of read IO
Is your concern performance, or that you are charged per IO?

Activate "pg_stat_statements" and turn on "track_io_timing".  Then you can look in "blk_read_time" column to find queries that spend a lot of time reading data from disk.  I recommend to look at the time column, not the count column "shared_blks_read", because a lot of the "reads" will actually be fulfilled by the OS's filesystem cache, so the count doesn't give you an accurate picture of true disk IO.  Timing should give a more accurate picture of that.

pg_stat_statements doesn't give you info on when queries are being run, other than 'since the last pg_stat_statements_reset() call'.  If you want to able to line the queries up with your chart, then you would want to enable "auto_explain", again after turning "track_io_timing" on, and adopt the settings of:

auto_explain.log_min_duration = '50ms' ## need to tweak for your needs
auto_explain.log_analyze=on
auto_explain.log_timing=off  ## this may have high overhead, so turn off
auto_explain.log_buffers=on


And then go mine the data out of the server log file.  This does come with an overhead, so you might to turn it off again once you find the problem.  The amount of overhead depends on your hardware and kernel, on modern systems I find it low enough to be worth paying to leave it on all the time, just cranking up "log_min_duration" to the point few queries get logged when I am not actively tracking a problem down.