[rrd-users] Newbie: "step" confusion, blank plots

[Mike Hunter]

Greetings,

I'm hoping that somebody can explain a little more about the "--step"
parameter to rrdcreate.  This is taken from "bigtops.pl" in
/usr/local/share/examples/rrdtool

...
RRDs::create ($rrd, "--start",$start-1, "--step",290,
              "DS:a:GAUGE:600:U:U",
              "DS:b:GAUGE:600:U:U",
              "RRA:AVERAGE:0.5:1:300");
my $ERROR = RRDs::error;
die "$0: unable to create `$rrd': $ERROR\n" if $ERROR;

# dropt some data into the rrd
my $t;
for ($t=$start; $t<$start+300*300; $t+=300){
...

So, if $t is gaining 300 seconds for each data point, why is step set to
290?  I noticed that if I change step to 220, the resulting picture is
shifted over to the right, but I don't understand why.

This all stems from me trying to do a simple graph of a data file that
looks like this:

2003-08-29.013001       10920070747     8291601450
2003-08-29.014501       9914236068      9615483756
2003-08-29.020001       8202305396      7795379166
2003-08-29.021501       7450926286      7995740622
2003-08-29.023001       7100548682      7943914961
2003-08-29.024501       7928561712      8222909469
2003-08-29.030001       5779334162      7954788420
2003-08-29.031501       5404870464      8294500672
2003-08-29.033001       4751317213      6861155522
2003-08-29.034501       5624335005      8089450973
2003-08-29.040001       2908052123      5655948510
2003-08-29.041501       5171003056      8099823516
2003-08-29.043001       4059829955      7416167984
2003-08-29.044501       4467995422      8396832232
2003-08-29.050001       3498427949      7258680374
2003-08-29.051501       4523794735      8226483341
2003-08-29.053001       3161418542      7720306661
2003-08-29.054501       3093824420      7140138469

I figured out how to convert the date string to unix epoch seconds, and I
tried to hack bigtops.pl to insert these time and data points instead of the 
ones it generates, but it produces blank graphs.  I tried hacking it to use 
the built-in 300 second time steps with my data points and *that* works...I'm 
breaking something by changing the time interval to the 900 seconds instead of 
300...but I tried 900 and 870 as "step" and I still got blankness....

Tschuss,

Mike Hunter

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