[rrd-users] Re: Base for AREA plots

[Alex van den Bogaerdt]

On Thu, Oct 21, 2004 at 02:14:32AM -0500, Christopher Snell wrote:
> I think my solution (posted in another thread) has the same problem: 
> values less than 0 will draw from 0 downwards instead of -20 upwards.
> 
> This RPN is burning my brain--it almost seems like it would be easier
> to patch RRDtool to enable alternate baselines, than to figure out the
> RPN required to do this.  :(

Start with
  DEF:temperature=   ...
  CDEF:minus20=temperature,POP,-20
  AREA:minus20
  STACK:temperature#color:legend
and remove any errors I made.

The "CDEF" statement uses "temperature,POP" to get an empty stack
but with a time/resolution component.  Then it pushes the number
-20 on this stack and ends.  This results in a constant of -20.

Next, AREA is used without a color; this results in a transparent
area.  On top of this invisible area, temperature is placed.

HTH
Alex
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