trace-graph: Limit task plot prev search by count not time
trace-graph: Limit task plot prev search by count not time

Since a time slot can hold thousands or just 1 event, search backwards
in the task plot and limit it by 20 events not time by pixels.

Signed-off-by: Steven Rostedt <43232e92d70cc7aa53504ad0397085ee47bad87f@goodmis.org>

bigzz/trace-cmd,bigzz/trace-cmd