[rrd-users] Re: Making sure no area overwrites another

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Thu Jan 3 00:41:18 MET 2002

Christohper Pope wrote:

> > This isn't going to work either because that Metro will win for the first
> > half of the graph, then the nitro's up and the Ferrari wins.  This results
> > in m overlaying f for the first half, then f overlaying m for the second half.
> > 
> At any point in time that is about to be graphed, rrdtool knows which car
> is going faster. I just want to graph its bar (AREA) first and then graph the
> bar for the slower one. This way the speed of the slower one will be graphed
> "ontop" of the faster one.

Right.  In that case the answer is: no.

>> [...stack...]
> This is kind of what I need, except like you said its the sum.

Sort of.  If it is in different colors you will know it is not one
speed.  It will be easy to see the difference in speed, it won't
be easy to find the actual speed on the y-axis for the second car.

Alternatives (some of which are hacks):

- Use lines, not areas.

- Draw one speed
- Draw an area which will certainly be positioned above the maximum
  speed of car m.  Make this line invisible by not specifying a color.
- Stack the other speed on top of the invisible area.
Result: two speeds always visible, y-axis is wrong for one of them.

- Create two different images and place them side by side.

- Wait for support of transparent colors, live with the mixing
  of colors.

- Make a clear definition of the correct behaviour of the areas,
  also define the proper user-interface to it.  Don't assume this
  mail conversation has been read and post a proposal to rrd-developers
  for comments.  If most of the people agree, Tobi specifically, you
  may find this in rrdtool, some time.

- Do it yourself and show us all the benefits.

- Have an extra DS available at twice the resolution of the speeds.
- Apply some CDEF magic and show the speed of car 1 at one half of
  each interval while showing the speed of car 2 at the other half.
This results in interlaced bars, maybe good enough for your needs.

- Use the negative part of the y-axis, assuming cars don't
  have negative speeds this may be good enough.  Again: CDEF.

 / alex at slot.hollandcasino.nl                  alex at ergens.op.het.net \
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