Cheshire Cat Computing

I notice when the target device is unreachable, the graph's current value remain unchanged as same as the last read value, Resulting in a flat line in the graph. Is it an expected behavior in MRTG? I thought it would show an empty line producing a gap effect in the graph which I was hoping to see. I am not integrated to RRDTool, just plain MRTG version 2.16.2 on Ubuntu Server.

I am using External Script to generate data, the script return correct value when the target is reachable (and the graph plots normally as expected).
13606672
53502192
uptime
myrouter

And I made my External Script returns "UNKNOWN" when the target is unreachable. It is when the graph's Current Value never change resulting in a flat line graph. Am I using the wrong value, should I return something different then "UNKNOWN"?
UNKNOWN
UNKNOWN
UNKNOWN
UNKNOWN

And this is the config calling an External Script: (I have tried deleting the gauge option, same result)

Options[myrouter]: growright, gauge
Target[myrouter]: `/home/echan/fetch_data.pl 10.42.83.99 .1.3.6.1.4.1.9.9.48.1.1.1.5.2 .1.3.6.1.4.1.9.9.48.1.1.1.6.2`
MaxBytes[myrouter]: 800000000
Title[myrouter]: Title2
PageTop[myrouter]: <H1>PageTop2</H1>
Timezone[myrouter]: Japan



MRTG returns following error when the target is unreachable which I believe is a expected behavior.
ERROR: Target[myrouter][_IN_] ' $target->[3]{$mode} ' did not eval into defined data

And I notice the latest value in the .log file is always -1 and it just keep copy over from the last known value.
1313779201 -1 -1
1313779201 6197316 6385596 6197316 6385596
1313778901 6197316 6385596 6197316 6385596
1313778900 6197316 6385596 6197316 6385596


Thanks.

When the target is unreachable, you should return 'UNKNOWN'.

Native-mode MRTG will display an unknown as the last known value -- resulting in the horizontal lines oyu mention. If thisis a problem, use the option 'unknaszero' to drop it to zero.

Routers2 will display an unknown as a grey area; you can use the option unknasprev to mimic the native-mode MRTG behaviour, or onknaszero (as in native mode).

_________________
Steve Shipway
UNIX Systems, ITSS, University of Auckland, NZ
Woe unto them that rise up early in the morning... -- Isaiah 5:11