ntpviz - Don't plot a line during data absence

[Jason Azze]

In the interest of keeping longer technical discussions on the devel list,
we have this:

Gary, Eric, and I had a discussion on #ntpsec about what I referred to as
"interpolation" performed by ntpviz when there is a hole in data
collection. Gary pointed out that it's really just gnuplot drawing lines
between data points.

<jasonium> I suppose gnuplot can't "know" the difference between a real
interval between points and one where something has broken.
<esr> That's probably it.  I expect you'd get a gap there in an impulse or
dot plot.
<esr> Might mean gemiller should tweak the plot mode.
<gemiller> jasonium: not exactly nptviz filling the blank spots, that is
gnuplot making lines between points.

There was some more discussion about adding runtime options to create
different kinds of graphs.

Then:
<gemiller> hmm, some people use up to poll 1024, I could preprocess the
plot file so that any gap longer than 1024 seconds does not get a
connecting line.  Sound good?  Or would people prefer to see just dots
first?
<jasonium> I like that compromise.

and:

<gemiller> jasonium:  I just pushed a change to ntpviz.  If no data for
over 1024 seconds, then tell gnuplot not to connect the lines.

and even later:

<gemiller> jasonium: further testing shows a 10% slowdown with my recent
hack.  So tell me if it is doing what you want before I try to regain some
of the lost time.  I have some ideas, but they will require some
contortions to the data set schema.
<gemiller> It was taking me 39 seconds to run a weeks logs on a RasPi3,
after the patch is is 43 second

I like the change. I've attached a sample image (I hope that's kosher on
the list), where you see peer "chilipepper" appear and disappear. In the
earlier version of ntpviz, gnuplot would have drawn straight lines between
"real" data points.

Whether or not it is worth a 10% slowdown is certainly debatable.

-- 
Jason Azze
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