In 1.6.7 if all stacked graph items have value of 0 during ALL selected period of display time - graph generation takes very long time then fails with timeout error
[warn] mod_fcgid: read data timeout in 40 seconds
[error] [client x.x.x.x] Premature end of script headers: chart2.php, referer charts.php?.....

the mod_fcgid presence is irrelevant, problem happens with mod_php as well.

If you expand viewing window to include just 1 nonzero value - it displays fine and it's nearly instant.

Probably some bug in graph scale calculation or something.

This was not happening in 1.4 AFAIK.

Update: only happens with zero values (uint in my case), it doesnt happend when there is no data for selected period.

HOW TO REPRODUCE (in my case):
create trapper uint graph item, set modifier to "speed per second", set multiplier to 60 so it would be speed mer minute.
Create stacked graph and insert this item into the graph.
Fill item history with 0-s for some period of time.

Browse graph, set graph window to the time interval that has only 0 values (no non-zero values).

The graph will not be displayed.

THis doesnt work with "latest data" graphs, those ones whow up just fine.