[hobbit] More Temperature Questions :-)

[Henrik Stoerner]

On Tue, Feb 15, 2005 at 11:49:55AM -0700, Charles Jones wrote:
> 1. How can I graph Farenheit instead of Celcius?

The easiest way of doing that is to change the hobbitgraph.cfg
definition for the graph!

In the [temperature] section, there's this line that picks out the
"temperature" setting from the RRD file:

    DEF:p at RRDIDX@=@RRDFN@:temperature:AVERAGE

This is an RRDtool "DEF" (definition) that defines a variable
"p at RRDIDX@" (the "@RRDIDX@" is handled by hobbitgraph, and is needed
because there are multiple graphs. So essentially you get p0, p1, p2
etc - one for each temperature*rrd file). 

There's also something called a "CDEF" - a Computed DEFinition.
Meaning you can do math on the data you have in the RRD. So to compute
Fahrenheit, you do the usual F = (9/5)*C + 32 - using Reverse Polish
Notation (RPN) which is what CDEF's understand:

   CDEF:tempf at RRDIDX@=9,5,/,p at RRDIDX@,*,32,+

Now you have two values: p at RRDIDX which is the original Celsius
temperature, and tempf at RRDIDX@ which is the Fahrenheit equivalent.
So to get the graph for the Fahrenheit one, change the LINE2 graph
definition to use the Fahrenheit value:

   LINE2:tempf at RRDIDX@#@COLOR@:@RRDPARAM@


Finally you just change the Y-axis label, and end up with this
definition:

[temperature]
        FNPATTERN temperature.(.*).rrd
        TITLE Temperature
        YAXIS Fahrenheit
        DEF:p at RRDIDX@=@RRDFN@:temperature:AVERAGE
        CDEF:tempf at RRDIDX@=9,5,/,p at RRDIDX@,*,32,+
        LINE2:tempf at RRDIDX@#@COLOR@:@RRDPARAM@
        GPRINT:p at RRDIDX@:LAST: \: %5.1lf (cur)
        GPRINT:p at RRDIDX@:MAX: \: %5.1lf (max)
        GPRINT:p at RRDIDX@:MIN: \: %5.1lf (min)
        GPRINT:p at RRDIDX@:AVERAGE: \: %5.1lf (avg)\n


Untested, but you get the idea ... the RRDtool website has some more
examples and tutorials on CDEF's.


Regards,
Henrik